Using RNA-guided FokI Nucleases (RFNs) to increase specificity for RNA-Guided Genome Editing

ABSTRACT

Many studies have shown that CRISPR-Cas nucleases can tolerate up to five mismatches and still cleave; it is hard to predict the effects of any given single or combination of mismatches on activity. Taken together, these nucleases can show significant off-target effects but it can be challenging to predict these sites. Described herein are methods for increasing the specificity of genome editing using the CRISPR/Cas system, e.g., using RNA-guided Foki Nucleases (RFNs), e.g., Fokl-Cas9 or Foki-dCas9-based fusion proteins.

CLAIM OF PRIORITY

This application is a national stage entry under 35 U.S.C. § 371 of PCT/US2014/035162, filed Apr. 23, 2014, and claims priority under 35 USC § 119(e) to U.S. Patent Application Ser. No. 61/838,178, filed on Jun. 21, 2013; 61/838,148, filed on Jun. 21, 2013; 61/921,007, filed on Dec. 26, 2013; and PCT/US2014/028630, filed Mar. 14, 2014. The entire contents of the foregoing are hereby incorporated by reference.

FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

This invention was made with Government support under Grant Nos. DP1 GM105378 awarded by the National Institutes of Health. The Government has certain rights in the invention.

TECHNICAL FIELD

Methods for increasing specificity of RNA-guided genome editing, e.g., editing using CRISPR/Cas9 systems, using RNA-guided FokI Nucleases (RFNs), e.g., FokI-dCas9 fusion proteins.

BACKGROUND

Recent work has demonstrated that clustered, regularly interspaced, short palindromic repeats (CRISPR)/CRISPR-associated (Cas) systems (Wiedenheft et al., Nature 482, 331-338 (2012); Horvath et al., Science 327, 167-170 (2010); Terns et al., Curr Opin Microbiol 14, 321-327 (2011)) can serve as the basis genome editing in bacteria, yeast and human cells, as well as in vivo in whole organisms such as fruit flies, zebrafish and mice (Wang et al., Cell 153, 910-918 (2013); Shen et al., Cell Res (2013); Dicarlo et al., Nucleic Acids Res (2013); Jiang et al., Nat Biotechnol 31, 233-239 (2013); Jinek et al., Elife 2, e00471 (2013); Hwang et al., Nat Biotechnol 31, 227-229 (2013); Cong et al., Science 339, 819-823 (2013); Mali et al., Science 339, 823-826 (2013c); Cho et al., Nat Biotechnol 31, 230-232 (2013); Gratz et al., Genetics 194(4):1029-35 (2013)). The Cas9 nuclease from S. pyogenes (hereafter simply Cas9) can be guided via base pair complementarity between the first 20 nucleotides of an engineered gRNA and the complementary strand of a target genomic DNA sequence of interest that lies next to a protospacer adjacent motif (PAM), e.g., a PAM matching the sequence NGG or NAG (Shen et al., Cell Res (2013); Dicarlo et al., Nucleic Acids Res (2013); Jiang et al., Nat Biotechnol 31, 233-239 (2013); Jinek et al., Elife 2, e00471 (2013); Hwang et al., Nat Biotechnol 31, 227-229 (2013); Cong et al., Science 339, 819-823 (2013); Mali et al., Science 339, 823-826 (2013c); Cho et al., Nat Biotechnol 31, 230-232 (2013); Jinek et al., Science 337, 816-821 (2012)). Previous studies performed in vitro (Jinek et al., Science 337, 816-821 (2012)), in bacteria (Jiang et al., Nat Biotechnol 31, 233-239 (2013)) and in human cells (Cong et al., Science 339, 819-823 (2013)) have shown that Cas9-mediated cleavage can, in some cases, be abolished by single mismatches at the gRNA/target site interface, particularly in the last 10-12 nucleotides (nts) located in the 3′ end of the 20 nucleotide (nt) gRNA complementarity region.

SUMMARY

Many studies have shown that CRISPR-Cas nucleases can tolerate up to five mismatches and still cleave; it is hard to predict the effects of any given single or combination of mismatches on activity. Taken together, these nucleases can show significant off-target effects but it can be challenging to predict these sites. Described herein are methods for increasing the specificity of genome editing using the CRISPR/Cas system, e.g., using RNA-guided FokI Nucleases (RFNs), e.g., FokI-Cas9 or FokI-dCas9-based fusion proteins.

In a first aspect, the invention provides FokI-dCas9 fusion proteins, comprising a FokI catalytic domain sequence fused to the terminus, e.g., the N terminus, of dCas9, optionally with an intervening linker, e.g., a linker of from 2-30 amino acids, e.g., 4-12 amino acids, e.g., Gly₄Ser. In some embodiments, the FokI catalytic domain comprises amino acids 388-583 or 408-583 of SEQ ID NO:4. In some embodiments, the dCas9 comprises mutations at the dCas9 comprises mutations at D10, E762, H983, or D986; and at H840 or N863; e.g., at: (i) D10A or D10N; and (ii) H840A, H840Y or H840N.

In another aspect, the invention provides nucleic acids encoding these fusion proteins, vector comprising the nucleic acids, and host cells harboring or expressing the nucleic acids, vectors, or fusion proteins.

In another aspect, the invention provides methods for inducing a sequence-specific break in a double-stranded DNA molecule, e.g., in a genomic sequence in a cell, the method comprising expressing in the cell, or contacting the cell with, the FokI-dCas9 fusion protein described herein, and:

(a) two single guide RNAs, wherein each of the two single guide RNAs include sequences that are each complementary to one strand of the target sequence such that using both guide RNAs results in targeting both strands (i.e., one single guide RNA targets a first strand, and the other guide RNA targets the complementary strand), and FokI cuts each strand resulting in a pair of nicks on opposite DNA strands, thereby creating a double-stranded break, or

(b) a tracrRNA and two crRNAs wherein each of the two crRNAs include sequences that are complementary to one strand of the target sequence such that using both crRNAs results in targeting both strands (i.e., one crRNA targets a first strand, and the other crRNA targets the complementary strand), and FokI cuts each strand resulting in a pair of nicks on opposite DNA strands, thereby creating a double-stranded break.

In another aspect, the invention provides methods for increasing specificity of RNA-guided genome editing in a cell, the method comprising contacting the cell with an RNA-guided FokI Nuclease (RFN) fusion protein described herein.

The method may further comprise expressing in the cell, or contacting the cell with, (a) two single guide RNAs, wherein each of the two single guide RNAs include sequences that are each complementary to one strand of the target sequence such that using both guide RNAs results in targeting both strands (i.e., one single guide RNA targets a first strand, and the other guide RNA targets the complementary strand), and FokI cuts each strand resulting in a pair of nicks on opposite DNA strands, thereby creating a double-stranded break, or

(b) a tracrRNA and two crRNAs wherein each of the two crRNAs include sequences that are complementary to one strand of the target sequence such that using both crRNAs results in targeting both strands (i.e., one crRNA targets a first strand, and the other crRNA targets the complementary strand), and FokI cuts each strand resulting in a pair of nicks on opposite DNA strands, thereby creating a double-stranded break.

In some embodiments, the two target genomic sequences (i.e., the sequences to which the target complementarity regions of the crRNA or single guide RNAs are complementary) are spaced 10-20 base pairs apart, preferably 13-17 base pairs apart.

In some embodiments, an indel mutation is induced between the two target sequences.

In some embodiments, the specificity of RNA-guided genome editing in a cell is increased.

Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. Methods and materials are described herein for use in the present invention; other, suitable methods and materials known in the art can also be used. The materials, methods, and examples are illustrative only and not intended to be limiting. All publications, patent applications, patents, sequences, database entries, and other references mentioned herein are incorporated by reference in their entirety. In case of conflict, the present specification, including definitions, will control.

Other features and advantages of the invention will be apparent from the following detailed description and figures, and from the claims.

SEQUENCE LISTING

The instant application contains a Sequence Listing which has been submitted electronically in ASCII format and is hereby incorporated by reference in its entirety. Said ASCII copy, created on Mar. 7, 2017, is named 40174_0008WO2_SL.txt and is 182,165 bytes in size.

DESCRIPTION OF DRAWINGS

FIG. 1: Schematic illustrating a gRNA/Cas9 nuclease complex bound to its target DNA site. Scissors indicate approximate cleavage points of the Cas9 nuclease on the genomic DNA target site (SEQ ID NO:286). Note the numbering of nucleotides on the guide RNA proceeds in an inverse fashion from 5′ to 3′.

FIG. 2A: Schematic illustrating the rationale for truncating the 5′ complementarity region of a gRNA. Thick grey lines=target DNA site, thin dark grey line structure=gRNA, grey oval=Cas9 nuclease, black lines indicate base pairing between gRNA and target DNA site.

FIG. 2B: Schematic overview of the EGFP disruption assay. Repair of targeted Cas9-mediated double-stranded breaks in a single integrated EGFP-PEST reporter gene by error-prone NHEJ-mediated repair leads to frame-shift mutations that disrupt the coding sequence and associated loss of fluorescence in cells.

FIGS. 2C-F: Activities of RGNs harboring sgRNAs bearing (C) single mismatches, (D) adjacent double mismatches, (E) variably spaced double mismatches, and (F) increasing numbers of adjacent mismatches assayed on three different target sites in the EGFP reporter gene sequence. Mean activities of replicates (see Online Methods) are shown, normalized to the activity of a perfectly matched gRNA. Error bars indicate standard errors of the mean. Positions mismatched in each gRNA are highlighted in grey in the grid below. Sequences of the three EGFP target sites were as follows:

EGFP Site 1 (SEQ ID NO: 1) GGGCACGGGCAGCTTGCCGGTGG  EGFP Site 2 (SEQ ID NO: 2) GATGCCGTTCTTCTGCTTGTCGG  EGFP Site 3 (SEQ ID NO: 3) GGTGGTGCAGATGAACTTCAGGG 

FIG. 2G: Mismatches at the 5′ end of the gRNA make CRISPR/Cas more sensitive more 3′ mismatches. The gRNAs Watson-Crick base pair between the RNA & DNA with the exception of positions indicated with an “m” which are mismatched using the Watson-Crick transversion (i.e. EGFP Site#2 M18-19 is mismatched by changing the gRNA to its Watson-Crick partner at positions 18 & 19. Although positions near the 5′ of the gRNA are generally very well tolerated, matches in these positions are important for nuclease activity when other residues are mismatched. When all four positions are mismatched, nuclease activity is no longer detectable. This further demonstrates that matches at these 5′ position can help compensate for mismatches at other more 3′ positions. Note these experiments were performed with a non-codon optimized version of Cas9 which can show lower absolute levels of nuclease activity as compared to the codon optimized version.

FIG. 2H: Efficiency of Cas9 nuclease activities directed by gRNAs bearing variable length complementarity regions ranging from 15 to 25 nts in a human cell-based U2OS EGFP disruption assay (SEQ ID NOs: 287-293, respectively). Expression of a gRNA from the U6 promoter requires the presence of a 5′ G and therefore it was only possible to evaluate gRNAs harboring certain lengths of complementarity to the target DNA site (15, 17, 19, 20, 21, 23, and 25 nts).

FIG. 3A: Efficiencies of EGFP disruption in human cells mediated by Cas9 and full-length or shortened gRNAs for four target sites in the EGFP reporter gene (SEQ ID NOs: 294-303, respectively). Lengths of complementarity regions and corresponding target DNA sites are shown. Ctrl=control gRNA lacking a complementarity region.

FIG. 3B: Efficiencies of targeted indel mutations introduced at seven different human endogenous gene targets by matched standard and tru-RGNs. Lengths of complementarity regions and corresponding target DNA sites are shown (SEQ ID NOs: 304-311, respectively). Indel frequencies were measured by T7EI assay. Ctrl=control gRNA lacking a complementarity region.

FIG. 3C: DNA sequences (SEQ ID NOs: 312-344, respectively) of indel mutations induced by RGNs using a tru-gRNA or a matched full-length gRNA targeted to the EMX1 site. The portion of the target DNA site that interacts with the gRNA complementarity region is highlighted in grey with the first base of the PAM sequence shown in lowercase. Deletions are indicated by dashes highlighted in grey and insertions by italicized letters highlighted in grey. The net number of bases deleted or inserted and the number of times each sequence was isolated are shown to the right.

FIG. 3D: Efficiencies of precise HDR/ssODN-mediated alterations introduced at two endogenous human genes by matched standard and tru-RGNs. % HDR was measured using a BamHI restriction digest assay (see the Experimental Procedures for Example 2). Control gRNA=empty U6 promoter vector.

FIG. 3E: U2OS.EGFP cells were transfected with variable amounts of full-length gRNA expression plasmids (top) or tru-gRNA expression plasmids (bottom) together with a fixed amount of Cas9 expression plasmid and then assayed for percentage of cells with decreased EGFP expression. Mean values from duplicate experiments are shown with standard errors of the mean. Note that the data obtained with tru-gRNA matches closely with data from experiments performed with full-length gRNA expression plasmids instead of tru-gRNA plasmids for these three EGFP target sites.

FIG. 3F: U2OS.EGFP cells were transfected with variable amount of Cas9 expression plasmid together with variable amounts of full-length gRNA expression plasmids (top) or tru-gRNA expression plasmids (bottom) (amounts determined for each tru-gRNA from the experiments of FIG. 3E). Mean values from duplicate experiments are shown with standard errors of the mean. Note that the data obtained with tru-gRNA matches closely with data from experiments performed with full-length gRNA expression plasmids instead of tru-gRNA plasmids for these three EGFP target sites. The results of these titrations determined the concentrations of plasmids used in the EGFP disruption assays performed in Examples 1 and 2.

FIGS. 4A-C. RNA-guided FokI nucleases and a CRISPR/Cas Subtype Ypest protein 4 (Csy4)-based multiplex gRNA expression system.

(a) Schematic overview of RNA-guided FokI nucleases. Two FokI-dCas9 fusion proteins are recruited to adjacent target sites by two different gRNAs in order to facilitate FokI dimerization and DNA cleavage.

(b) Schematic overview of a Csy4-based multiplex gRNA expression system. Two gRNAs (with any 5′ end nucleotide) are co-expressed in a single transcript from a U6 promoter with each gRNA flanked by Csy4 recognition sites. Csy4 cleaves and releases gRNAs from the transcript. The Csy4 recognition site remains at the 3′ end of the gRNA with a Csy4 nuclease bound to that site.

(c) Validation of the multiplex, Csy4-based system. Two gRNAs targeted to adjacent sites in EGFP were expressed in a single RNA transcript using the Csy4-based system in human U2OS.EGFP cells together with Csy4 and Cas9 nucleases. Sequences of indel mutations induced in these cells are shown. The wild-type sequence is shown in the top with both target sites highlighted in grey and PAM sequences shown as underlined text. Deletions are indicated by dashes against gray background and insertions by lowercase letters against a grey background. To the right of each sequence, the sizes of insertions (+) or deletions (Δ) are specified.

FIGS. 5A-F. Design and optimization of RNA-guided FokI nucleases.

(a) Schematic illustrations of a ZFN, TALEN, FokI-dCas9 fusion, and dCas9-FokI fusion.

(b) Screening the EGFP disruption activities of FokI-dCas9 fusion with gRNA pairs targeted to half-sites in one of two orientations: PAMs in (left panel) and PAMs out (right panel). Half-sites were separated by spacer sequences of variable lengths ranging from 0 to 31 bps. EGFP disruption was quantified by flow cytometry, n=1. Corresponding data for the dCas9-FokI fusion and the same gRNA pairs is shown in FIG. 5E.

(c) Additional assessment of FokI-dCas9-mediated EGFP disruption activities on target sites with half-sites oriented with their PAMs out and with spacer lengths ranging from 10 to 20 bp. EGFP disruption was quantified by flow cytometry. Error bars indicate standard errors of the mean (s.e.m.), n=2.

(d) Mean EGFP disruption values of the data from (c) grouped according to spacer length. Error bars represent s.e.m.

(e) These plots show the results of a screen for dCas9-FokI activity in EGFP disruption assay in the U2OS.EGFP cells with 60 gRNA pairs with spacings of 0-31 bp and PAM in and PAM out orientations.

(f) Sequences of FokI-dCas9 induced mutations in U2OS cells are shown (SEQ ID NOs: 354-466, respectively, in order of appearance). The 23-nt target sequence bound by Cas9 or FokI-dCas9 is labeled in grey. The protospacer adjacent motif or PAM sequence is labeled in boldface with underlining. Deletions are marked with dashes on a light grey background. Insertions are highlighted in grey. The net number of bases inserted or deleted are indicated in a column directly to the right of the sequences.

FIGS. 6A-D. Dimerization of FokI-dCas9 RFNs is required for efficient genome editing activity.

(a) EGFP disruption activities of two RFN pairs assessed in the presence of correctly targeted gRNA pairs (to EGFP sites 47 and 81) and pairs in which one or the other of the gRNAs has been replaced with another gRNA targeted to a non-EGFP sequence (in the VEGFA gene). EGFP disruption was quantified by flow cytometry. EGFP, Enhanced Green Fluorescent Protein; VEGFA, Vascular Endothelial Growth Factor A. Error bars represent standard errors of the mean (s.e.m.), n=3.

(b) Quantification of mutagenesis frequencies by T7EI assay performed with genomic DNA from the same cells used in the EGFP disruption assay of (a). Error bars represent s.e.m., n=3.

(c) Activities of RFNs targeted to sites in the APC, MLH1 and VEGFA genes. For each target, we co-expressed FokI-dCas9 with a pair of cognate gRNAs, only one gRNA for the “left” half-site, or only one gRNA for the “right” half-site. Rates of mutagenesis were measured by T7E1 assay. APC, Adenomatous polyposis coli; MLH1, mutL homolog 1; VEGFA, Vascular Endothelial Growth Factor A. Error bars represent s.e.m., n=3.

(d) Mutagenesis frequencies of RFNs targeted to VEGFA site 1 at the on-target site and at five previously known off-target (OT) sites for one of the gRNAs used to target VEGFA site 1. Frequencies of mutation were determined by deep sequencing. Each value reported was determined from a single deep sequencing library prepared from genomic DNA pooled from three independent transfection experiments. The value shown for the on-target VEGFA site 1 (marked with an asterisk) is the same as the one shown in FIG. 4a below and is only shown here for ease of comparison with the values presented in this figure.

FIGS. 7A-B. Mutagenic activities of a Cas9 nickase or FokI-dCas9 co-expressed with a single gRNA.

(a) Indel mutation frequencies induced by FokI-dCas9 (left bars) or Cas9 nickase (middle bars) in the presence of one or two gRNAs targeted to six different human gene sites. For each gene target, we assessed indel frequencies with both gRNAs, only one gRNA for the “left” half-site, or only the other gRNA for the “right” half-site. Mutation frequencies were determined by deep sequencing. Each indel frequency value reported was determined from a single deep sequencing library prepared from genomic DNA pooled from three independent transfection experiments. VEGFA, Vascular Endothelial Growth Factor A; DDB2, Damage-Specific DNA Binding Protein 2; FANCF, Fanconi Anemia, Complementation Group F; FES, Feline Sarcoma Oncogene; RUNX 1, Runt-Related Transcription Factor 1.

(b) Data from (a) presented as a fold-reduction in the indel frequency comparing values obtained for each target site with a gRNA pair to each of the single gRNA experiments or to the control experiment (no gRNA and no Cas9 nickase or FokI-dCas9). This fold-reduction was calculated for both FokI-dCas9 (left bars in each pair, lighter grey) and Cas9 nickase (right bars in each pair, darker grey).

FIGS. 8A-C: Single Cas9 nickases can introduce point mutations with high efficiencies into their target sites.

Frequencies of different point mutations found at each position in half-sites targeted by single gRNAs for (a) VEGFA, (b) FANCF, and (c) RUNX1 gene targets in the presence of FokI-dCas9, Cas9 nickase, or a tdTomato control. Mutation frequencies were determined by deep sequencing. Each point mutation value reported was determined from a single deep sequencing library prepared from genomic DNA pooled from three independent transfection experiments. Note that the genomic DNA used for these experiments was isolated from the same cells analyzed for indel mutations in FIGS. 7A-B. VEGFA, Vascular Endothelial Growth Factor A; FANCF, Fanconi Anemia, Complementation Group F; RUNX 1, Runt-Related Transcription Factor 1.

DETAILED DESCRIPTION

CRISPR RNA-guided nucleases (RGNs) have rapidly emerged as a facile and efficient platform for genome editing. Although Marraffini and colleagues (Jiang et al., Nat Biotechnol 31, 233-239 (2013)) recently performed a systematic investigation of Cas9 RGN specificity in bacteria, the specificities of RGNs in human cells have not been extensively defined. Understanding the scope of RGN-mediated off-target effects in human and other eukaryotic cells will be critically essential if these nucleases are to be used widely for research and therapeutic applications. The present inventors have used a human cell-based reporter assay to characterize off-target cleavage of Cas9-based RGNs. Single and double mismatches were tolerated to varying degrees depending on their position along the guide RNA (gRNA)-DNA interface. Off-target alterations induced by four out of six RGNs targeted to endogenous loci in human cells were readily detected by examination of partially mismatched sites. The off-target sites identified harbor up to five mismatches and many are mutagenized with frequencies comparable to (or higher than) those observed at the intended on-target site. Thus RGNs are highly active even with imperfectly matched RNA-DNA interfaces in human cells, a finding that might confound their use in research and therapeutic applications.

The results described herein reveal that predicting the specificity profile of any given RGN is neither simple nor straightforward. The EGFP reporter assay experiments show that single and double mismatches can have variable effects on RGN activity in human cells that do not strictly depend upon their position(s) within the target site. For example, consistent with previously published reports, alterations in the 3′ half of the gRNA/DNA interface generally have greater effects than those in the 5′ half (Jiang et al., Nat Biotechnol 31, 233-239 (2013); Cong et al., Science 339, 819-823 (2013); Jinek et al., Science 337, 816-821 (2012)); however, single and double mutations in the 3′ end sometimes also appear to be well tolerated whereas double mutations in the 5′ end can greatly diminish activities. In addition, the magnitude of these effects for mismatches at any given position(s) appears to be site-dependent. Comprehensive profiling of a large series of RGNs with testing of all possible nucleotide substitutions (beyond the Watson-Crick transversions used in our EGFP reporter experiments) may help provide additional insights into the range of potential off-targets. In this regard, the recently described bacterial cell-based method of Marraffini and colleagues (Jiang et al., Nat Biotechnol 31, 233-239 (2013)) or the in vitro, combinatorial library-based cleavage site-selection methodologies previously applied to ZFNs by Liu and colleagues (Pattanayak et al., Nat Methods 8, 765-770 (2011)) might be useful for generating larger sets of RGN specificity profiles.

Despite these challenges in comprehensively predicting RGN specificities, it was possible to identify bona fide off-targets of RGNs by examining a subset of genomic sites that differed from the on-target site by one to five mismatches. Notably, under conditions of these experiments, the frequencies of RGN-induced mutations at many of these off-target sites were similar to (or higher than) those observed at the intended on-target site, enabling the detection of mutations at these sites using the T7EI assay (which, as performed in our laboratory, has a reliable detection limit of ˜2 to 5% mutation frequency). Because these mutation rates were very high, it was possible to avoid using deep sequencing methods previously required to detect much lower frequency ZFN- and TALEN-induced off-target alterations (Pattanayak et al., Nat Methods 8, 765-770 (2011); Perez et al., Nat Biotechnol 26, 808-816 (2008); Gabriel et al., Nat Biotechnol 29, 816-823 (2011); Hockemeyer et al., Nat Biotechnol 29, 731-734 (2011)). Analysis of RGN off-target mutagenesis in human cells also confirmed the difficulties of predicting RGN specificities—not all single and double mismatched off-target sites show evidence of mutation whereas some sites with as many as five mismatches can also show alterations. Furthermore, the bona fide off-target sites identified do not exhibit any obvious bias toward transition or transversion differences relative to the intended target sequence.

Although off-target sites were seen for a number of RGNs, identification of these sites was neither comprehensive nor genome-wide in scale. For the six RGNs studied, only a very small subset of the much larger total number of potential off-target sequences in the human genome was examined. Although examining such large numbers of loci for off-target mutations by T7EI assay is neither a practical nor a cost-effective strategy, the use of high-throughput sequencing in future studies might enable the interrogation of larger numbers of candidate off-target sites and provide a more sensitive method for detecting bona fide off-target mutations. For example, such an approach might enable the unveiling of additional off-target sites for the two RGNs for which we failed to uncover any off-target mutations. In addition, an improved understanding both of RGN specificities and of any epigenomic factors (e.g., DNA methylation and chromatin status) that may influence RGN activities in cells might also reduce the number of potential sites that need to be examined and thereby make genome-wide assessments of RGN off-targets more practical and affordable.

A number of strategies can be used to minimize the frequencies of genomic off-target mutations. For example, the specific choice of RGN target site can be optimized; given that off-target sites that differ at up to five positions from the intended target site can be efficiently mutated by RGNs, choosing target sites with minimal numbers of off-target sites as judged by mismatch counting seems unlikely to be effective; thousands of potential off-target sites that differ by four or five positions within the 20 bp RNA:DNA complementarity region will typically exist for any given RGN targeted to a sequence in the human genome. It is also possible that the nucleotide content of the gRNA complementarity region might influence the range of potential off-target effects. For example, high GC-content has been shown to stabilize RNA:DNA hybrids (Sugimoto et al., Biochemistry 34, 11211-11216 (1995)) and therefore might also be expected to make gRNA/genomic DNA hybridization more stable and more tolerant to mismatches. Additional experiments with larger numbers of gRNAs will be needed to assess if and how these two parameters (numbers of mismatched sites in the genome and stability of the RNA:DNA hybrid) influence the genome-wide specificities of RGNs. However, it is important to note that even if such predictive parameters can be defined, the effect of implementing such guidelines would be to further restrict the targeting range of RGNs.

One potential general strategy for reducing RGN-induced off-target effects might be to reduce the concentrations of gRNA and Cas9 nuclease expressed in the cell. This idea was tested using the RGNs for VEGFA target sites 2 and 3 in U2OS.EGFP cells; transfecting less gRNA- and Cas9-expressing plasmid decreased the mutation rate at the on-target site but did not appreciably change the relative rates of off-target mutations. Consistent with this, high-level off-target mutagenesis rates were also observed in two other human cell types (HEK293 and K562 cells) even though the absolute rates of on-target mutagenesis are lower than in U2OS.EGFP cells. Thus, reducing expression levels of gRNA and Cas9 in cells is not likely to provide a solution for reducing off-target effects. Furthermore, these results also suggest that the high rates of off-target mutagenesis observed in human cells are not caused by overexpression of gRNA and/or Cas9.

The finding that significant off-target mutagenesis can be induced by RGNs in three different human cell types has important implications for broader use of this genome-editing platform. For research applications, the potentially confounding effects of high frequency off-target mutations will need to be considered, particularly for experiments involving either cultured cells or organisms with slow generation times for which the outcrossing of undesired alterations would be challenging. One way to control for such effects might be to utilize multiple RGNs targeted to different DNA sequences to induce the same genomic alteration because off-target effects are not random but instead related to the targeted site. However, for therapeutic applications, these findings clearly indicate that the specificities of RGNs will need to be carefully defined and/or improved if these nucleases are to be used safely in the longer term for treatment of human diseases.

Methods for Improving Specificity

As shown herein, CRISPR-Cas RNA-guided nucleases based on the S. pyogenes Cas9 protein can have significant off-target mutagenic effects that are comparable to or higher than the intended on-target activity (Example 1). Such off-target effects can be problematic for research and in particular for potential therapeutic applications. Therefore, methods for improving the specificity of CRISPR-Cas RNA guided nucleases (RGNs) are needed.

As described in Example 1, Cas9 RGNs can induce high-frequency indel mutations at off-target sites in human cells (see also Cradick et al., 2013; Fu et al., 2013; Hsu et al., 2013; Pattanayak et al., 2013). These undesired alterations can occur at genomic sequences that differ by as many as five mismatches from the intended on-target site (see Example 1). In addition, although mismatches at the 5′ end of the gRNA complementarity region are generally better tolerated than those at the 3′ end, these associations are not absolute and show site-to-site-dependence (see Example 1 and Fu et al., 2013; Hsu et al., 2013; Pattanayak et al., 2013). As a result, computational methods that rely on the number and/or positions of mismatches currently have limited predictive value for identifying bona fide off-target sites. Therefore, methods for reducing the frequencies of off-target mutations remain an important priority if RNA-guided nucleases are to be used for research and therapeutic applications.

Dimerization is an attractive potential strategy for improving the specificity of Cas9 nucleases. This is distinct from a paired Cas9 nickase approach, which is not a true dimeric system. Paired nickases work by co-localizing two Cas9 nickases on a segment of DNA, thereby inducing high efficiency genome editing via an undefined mechanism. Because dimerization is not required for enzymatic activity, single Cas9 nickases can also induce indels with high efficiencies at certain sites (via an unknown mechanism) and can therefore potentially cause unwanted off-target mutations in the genome.

Thus, one strategy to improve the specificity of RGNs is fusing a FokI endonuclease domain to a catalytically inactive form of Cas9 bearing the D10A and H840A mutations (also known as dCas9). FokI nuclease domain functions as a dimer and therefore two subunits must be recruited to the same local piece of DNA in order to induce a double-stranded break. In this configuration (FIG. 9A and Example 2), two FokI-dCas9 fusions are recruited in an appropriate configuration using two different gRNAs to yield a double-stranded break. Thus, in this system, the FokI-dCas9 fusions would bind to a site that is twice as long as that of a single RGN and therefore this system would be expected to be more specific.

Therefore provided herein are FokI-dCas9 fusion proteins, wherein the FokI sequence is fused to dCas9 (preferably to the amino-terminal end of dCas9, but also optionally to the carboxy terminus), optionally with an intervening linker, e.g., a linker of from 2-30 amino acids, e.g., 4-12 amino acids, e.g., Gly₄Ser (SEQ ID NO:23) or (Gly₄Ser)₃. In some embodiments, the fusion proteins include a linker between the dCas9 and the FokI domains. Linkers that can be used in these fusion proteins (or between fusion proteins in a concatenated structure) can include any sequence that does not interfere with the function of the fusion proteins. In preferred embodiments, the linkers are short, e.g., 2-20 amino acids, and are typically flexible (i.e., comprising amino acids with a high degree of freedom such as glycine, alanine, and serine). In some embodiments, the linker comprises one or more units consisting of GGGS (SEQ ID NO:22) or GGGGS (SEQ ID NO:23), e.g., two, three, four, or more repeats of the GGGS (SEQ ID NO:22) or GGGGS (SEQ ID NO:23) unit. Other linker sequences can also be used.

Also described herein is a RNA-guided FokI nuclease platform in which dimerization, rather than just co-localization, is required for efficient genome editing activity. These nucleases can robustly mediate highly efficient genome editing in human cells and can reduce off-target mutations to undetectable levels as judged by sensitive deep sequencing methods. Also described is an efficient system for expressing pairs of gRNAs with any 5′ end nucleotide, a method that confers a wider targeting range on the RFN platform. Finally, monomeric Cas9 nickases generally introduce more undesirable indels and point mutations than the nucleases described herein in the presence of a single gRNA. These results define a robust, user-friendly nuclease platform with the specificity advantages of a well-characterized dimeric architecture and an improved mutagenesis profile relative to paired Cas9 nickases, features that will be important for research or therapeutic applications requiring the highest possible genome editing precision.

Thus a new RNA-guided FokI Nuclease (RFN) platform is described herein for performing robust and highly specific genome editing in human cells. RFNs require two gRNAs for activity and function as dimers. Surprisingly, the engineering of an active RFN required fusion of the FokI nuclease domain to the amino-terminal end of the dCas9 protein, an architecture different from ZFNs and TALENs in which the FokI domain is fused to the carboxy-terminal end of engineered zinc finger or transcription activator-like effector repeat arrays. RFNs also require that the half-sites bound by each Fok-dCas9/gRNA complex have a particular relative orientation (PAMs out) with a relatively restricted intervening spacer length of 14 to 17 bps (although activity may be possible at additional spacings but with less consistent success).

The dimeric nature of RFNs provides important specificity advantages relative to standard monomeric Cas9 nucleases. In an ideal dimeric system, little to no activity will be observed with monomers on half-sites. The present data demonstrate that FokI-dCas9 directed by a single gRNA induces very little or no mutagenesis at RFN half-sites. 12 single gRNAs (for six RFN target sites) were tested with co-expressed FokI-dCas9 and indels were observed at very low frequencies (range of 0.0045% to 0.47%), in some cases at levels as low as background rates observed in control cells in which there was no expression of gRNA or nuclease. Given that the FokI nuclease domain functions as a dimer, it is presumed that any indels observed with a single gRNA are likely due to recruitment of a FokI-dCas9 dimer to the DNA. Regardless of mechanism, given that only very low level mutagenesis was observed when FokI-dCas9 was tested with single gRNAs at 12 on-target half-sites, it is very unlikely that any mutagenesis will be induced at partially mismatched, off-target half-sites. Indeed, an RFN targeted to VEGFA did not induce detectable mutations at known off-target sites of one of the gRNAs as judged by deep sequencing.

Because RFNs are a true dimeric system, they possess a number of important advantages over paired nickase technology, which depends on co-localization but does not require dimerization. First, the direct comparisons herein show that single Cas9 nickases generally induce indel mutations with greater efficiencies than do FokI-dCas9 fusion proteins directed by the same individual gRNAs. Second, monomeric Cas9 nickases can also induce base pair substitutions in their target half-sites with high efficiencies, a previously unknown mutagenic side-effect that we uncovered in this study. Again, the direct comparisons show that monomeric Cas9 nickases induce these unwanted point mutations at substantially higher rates than FokI-dCas9 fusions guided by the same single gRNAs. Third, paired Cas9 nickases show greater promiscuity in the orientation and spacing of target half-sites than dimeric RFNs and therefore have a greater potential range of sites at which off-target mutations might be induced. Paired nickase half-sites can be oriented with their PAMs in or PAMs out and with spacer sequences ranging in length from 0 to 1000 bps (Ran et al., Cell 154, 1380-1389 (2013); Mali et al., Nat Biotechnol 31, 833-838 (2013); Cho et al., Genome Res (2013)). This promiscuity exists because the genome editing activities of Cas9 nickases do not depend on dimerization of the enzyme but rather just co-localization of the two nicks. By contrast, RFNs are much more stringent in their specificities—half-sites must have their PAMs out and must be spaced apart by 14 to 17 bps, due to the requirement for two appropriately positioned FokI cleavage domains for efficient cleavage.

FokI

FokI is a type IIs restriction endonuclease that includes a DNA recognition domain and a catalytic (endonuclease) domain. The fusion proteins described herein can include all of FokI or just the catalytic endonuclease domain, e.g., amino acids 388-583 or 408-583 of GenBank Acc. No. AAA24927.1, e.g., as described in Li et al., Nucleic Acids Res. 39(1): 359-372 (2011); Cathomen and Joung, Mol. Ther. 16: 1200-1207 (2008), or a mutated form of FokI as described in Miller et al. Nat Biotechnol 25: 778-785 (2007); Szczepek et al., Nat Biotechnol 25: 786-793 (2007); or Bitinaite et al., Proc. Natl. Acad. Sci. USA. 95:10570-10575 (1998).

An exemplary amino acid sequence of FokI is as follows:

(SEQ ID NO: 4)         10         20         30         40  MFLSMVSKIR TFGWVQNPGK FENLKRVVQV FDRNSKVHNE         50         60         70         80 VKNIKIPTLV KESKIQKELV AIMNQHDLIY TYKELVGTGT         90        100        110        120 SIRSEAPCDA IIQATIADQG NKKGYIDNWS SDGFLRWAHA        130        140        150        160 LGFIEYINKS DSFVITDVGL AYSKSADGSA IEKEILIEAI        170        180        190        200 SSYPPAIRIL TLLEDGQHLT KFDLGKNLGF SGESGFTSLP         210       220        230        240 EGILLDTLAN AMPKDKGEIR NNWEGSSDKY ARMIGGWLDK        250        260        270        280 LGLVKQGKKE FIIPTLGKPD NKEFISHAFK ITGEGLKVLR        290        300        310        320 RAKGSTKFTR VPKRVYWEML ATNLTDKEYV RTRRALILEI        330        340        350        360 LIKAGSLKIE QIQDNLKKLG FDEVIETIEN DIKGLINTGI        370        380        390        400  FIEIKGRFYQ LKDHILQFVI PNRGVTKQLV KSELEEKKSE        410        420        430        440 LRHKLKYVPH EYIELIEIAR NSTQDRILEM KVMEFFMKVY        450        460        470        480 GYRGKHLGGS RKPDGAIYTV GSPIDYGVIV DTKAYSGGYN        490        500        510        520  LPIGQADEMQ RYVEENQTRN KHINPNEWWK VYPSSVTEFK        530        540        550        560 FLFVSGHFKG NYKAQLTRLN HITNCNGAVL SVEELLIGGE 570 580 MIKAGTLTLE EVRRKFNNGE INF 

An exemplary nucleic acid sequence encoding FokI is as follows:

(SEQ ID NO: 285) ATGTTTTTGAGTATGGTTTCTAAAATAAGAACTTTCGGTTGGGTTCAAA ATCCAGGTAAATTTGAGAATTTAAAACGAGTAGTTCAAGTATTTGATAG AAATTCTAAAGTACATAATGAAGTGAAAAATATAAAGATACCAACCCTA GTCAAAGAAAGTAAGATCCAAAAAGAACTAGTTGCTATTATGAATCAAC ATGATTTGATTTATACATATAAAGAGTTAGTAGGAACAGGAACTTCAAT ACGTTCAGAAGCACCATGCGATGCAATTATTCAAGCAACAATAGCAGAT CAAGGAAATAAAAAAGGCTATATCGATAATTGGTCATCTGACGGTTTTT TGCGTTGGGCACATGCTTTAGGATTTATTGAATATATAAATAAAAGTGA TTCTTTTGTAATAACTGATGTTGGACTTGCTTACTCTAAATCAGCTGAC GGCAGCGCCATTGAAAAAGAGATTTTGATTGAAGCGATATCATCTTATC CTCCAGCGATTCGTATTTTAACTTTGCTAGAAGATGGACAACATTTGAC AAAGTTTGATCTTGGCAAGAATTTAGGTTTTAGTGGAGAAAGTGGATTT ACTTCTCTACCGGAAGGAATTCTTTTAGATACTCTAGCTAATGCTATGC CTAAAGATAAAGGCGAAATTCGTAATAATTGGGAAGGATCTTCAGATAA GTACGCAAGAATGATAGGTGGTTGGCTGGATAAACTAGGATTAGTAAAG CAAGGAAAAAAAGAATTTATCATTCCTACTTTGGGTAAGCCGGACAATA AAGAGTTTATATCCCACGCTTTTAAAATTACTGGAGAAGGTTTGAAAGT ACTGCGTCGAGCAAAAGGCTCTACAAAATTTACACGTGTACCTAAAAGA GTATATTGGGAAATGCTTGCTACAAACCTAACCGATAAAGAGTATGTAA GAACAAGAAGAGCTTTGATTTTAGAAATATTAATCAAAGCTGGATCATT AAAAATAGAACAAATACAAGACAACTTGAAGAAATTAGGATTTGATGAA GTTATAGAAACTATTGAAAATGATATCAAAGGCTTAATTAACACAGGTA TATTTATAGAAATCAAAGGGCGATTTTATCAATTGAAAGACCATATTCT TCAATTTGTAATACCTAATCGTGGTGTGACTAAGCAACTAGTCAAAAGT GAACTGGAGGAGAAGAAATCTGAACTTCGTCATAAATTGAAATATGTGC CTCATGAATATATTGAATTAATTGAAATTGCCAGAAATTCCACTCAGGA TAGAATTCTTGAAATGAAGGTAATGGAATTTTTTATGAAAGTTTATGGA TATAGAGGTAAACATTTGGGTGGATCAAGGAAACCGGACGGAGCAATTT ATACTGTCGGATCTCCTATTGATTACGGTGTGATCGTGGATACTAAAGC TTATAGCGGAGGTTATAATCTGCCAATTGGCCAAGCAGATGAAATGCAA CGATATGTCGAAGAAAATCAAACACGAAACAAACATATCAACCCTAATG AATGGTGGAAAGTCTATCCATCTTCTGTAACGGAATTTAAGTTTTTATT TGTGAGTGGTCACTTTAAAGGAAACTACAAAGCTCAGCTTACACGATTA AATCATATCACTAATTGTAATGGAGCTGTTCTTAGTGTAGAAGAGCTTT TAATTGGTGGAGAAATGATTAAAGCCGGCACATTAACCTTAGAGGAAGT GAGACGGAAATTTAATAACGGCGAGATAAACTTTTAA 

In some embodiments, the FokI nuclease used herein is at least about 50% identical SEQ ID NO:4, e.g., to amino acids 388-583 or 408-583 of SEQ ID NO:4. These variant nucleases must retain the ability to cleave DNA. In some embodiments, the nucleotide sequences are about 50%, 55%, 60%, 65%, 70%, 75%, 80%, 85%, 90%, 95%, 99% or 100% identical to amino acids 388-583 or 408-583 of SEQ ID NO:4. In some embodiments, any differences from amino acids 388-583 or 408-583 of SEQ ID NO:4 are in non-conserved regions.

To determine the percent identity of two sequences, the sequences are aligned for optimal comparison purposes (gaps are introduced in one or both of a first and a second amino acid or nucleic acid sequence as required for optimal alignment, and non-homologous sequences can be disregarded for comparison purposes). The length of a reference sequence aligned for comparison purposes is at least 50% (in some embodiments, about 50%, 55%, 60%, 65%, 70%, 75%, 85%, 90%, 95%, or 100% of the length of the reference sequence is aligned). The nucleotides or residues at corresponding positions are then compared. When a position in the first sequence is occupied by the same nucleotide or residue as the corresponding position in the second sequence, then the molecules are identical at that position. The percent identity between the two sequences is a function of the number of identical positions shared by the sequences, taking into account the number of gaps, and the length of each gap, which need to be introduced for optimal alignment of the two sequences.

The comparison of sequences and determination of percent identity between two sequences can be accomplished using a mathematical algorithm. For purposes of the present application, the percent identity between two amino acid sequences is determined using the Needleman and Wunsch ((1970) J. Mol. Biol. 48:444-453) algorithm which has been incorporated into the GAP program in the GCG software package, using a Blossum 62 scoring matrix with a gap penalty of 12, a gap extend penalty of 4, and a frameshift gap penalty of 5.

Cas9

A number of bacteria express Cas9 protein variants. The Cas9 from Streptococcus pyogenes is presently the most commonly used; some of the other Cas9 proteins have high levels of sequence identity with the S. pyogenes Cas9 and use the same guide RNAs. Others are more diverse, use different gRNAs, and recognize different PAM sequences as well (the 2-5 nucleotide sequence specified by the protein which is adjacent to the sequence specified by the RNA). Chylinski et al. classified Cas9 proteins from a large group of bacteria (RNA Biology 10:5, 1-12; 2013), and a large number of Cas9 proteins are listed in supplementary FIG. 1 and supplementary table 1 thereof, which are incorporated by reference herein. Additional Cas9 proteins are described in Esvelt et al., Nat Methods. 2013 November; 10(11):1116-21 and Fonfara et al., “Phylogeny of Cas9 determines functional exchangeability of dual-RNA and Cas9 among orthologous type II CRISPR-Cas systems.” Nucleic Acids Res. 2013 Nov. 22. [Epub ahead of print] doi:10.1093/nar/gkt1074.

Cas9 molecules of a variety of species can be used in the methods and compositions described herein. While the S. pyogenes and S. thermophilus Cas9 molecules are the subject of much of the disclosure herein, Cas9 molecules of, derived from, or based on the Cas9 proteins of other species listed herein can be used as well. In other words, while the much of the description herein uses S. pyogenes and S. thermophilus Cas9 molecules, Cas9 molecules from the other species can replace them. Such species include those set forth in the following table, which was created based on supplementary FIG. 1 of Chylinski et al., 2013.

Alternative Cas9 proteins GenBank Acc No. Bacterium 303229466 Veillonella atypica ACS-134-V-Col7a 34762592 Fusobacterium nucleatum subsp. vincentii 374307738 Filifactor alocis ATCC 35896 320528778 Solobacterium moorei F0204 291520705 Coprococcus catus GD-7 42525843 Treponema denticola ATCC 35405 304438954 Peptoniphilus duerdenii ATCC BAA-1640 224543312 Catenibacterium mitsuokai DSM 15897 24379809 Streptococcus mutans UA159 15675041 Streptococcus pyogenes SF370 16801805 Listeria innocua Clip11262 116628213 Streptococcus thermophilus LMD-9 323463801 Staphylococcus pseudintermedius ED99 352684361 Acidaminococcus intestini RyC-MR95 302336020 Olsenella uli DSM 7084 366983953 Oenococcus kitaharae DSM 17330 310286728 Bifidobacterium bifidum S17 258509199 Lactobacillus rhamnosus GG 300361537 Lactobacillus gasseri JV-V03 169823755 Finegoldia magna ATCC 29328 47458868 Mycoplasma mobile 163K 284931710 Mycoplasma gallisepticum str. F 363542550 Mycoplasma ovipneumoniae SC01 384393286 Mycoplasma canis PG 14 71894592 Mycoplasma synoviae 53 238924075 Eubacterium rectale ATCC 33656 116627542 Streptococcus thermophilus LMD-9 315149830 Enterococcus faecalis TX0012 315659848 Staphylococcus lugdunensis M23590 160915782 Eubacterium dolichum DSM 3991 336393381 Lactobacillus coryniformis subsp. torquens 310780384 Ilyobacter polytropus DSM 2926 325677756 Ruminococcus albus 8 187736489 Akkermansia muciniphila ATCC BAA-835 117929158 Acidothermus cellulolyticus 11B 189440764 Bifidobacterium longum DJO10A 283456135 Bifidobacterium dentium Bd1 38232678 Corynebacterium diphtheriae NCTC 13129 187250660 Elusimicrobium minutum Pei191 319957206 Nitratifractor salsuginis DSM 16511 325972003 Sphaerochaeta globus str. Buddy 261414553 Fibrobacter succinogenes subsp. succinogenes 60683389 Bacteroides fragilis NCTC 9343 256819408 Capnocytophaga ochracea DSM 7271 90425961 Rhodopseudomonas palustris BisB18 373501184 Prevotella micans F0438 294674019 Prevotella ruminicola 23 365959402 Flavobacterium columnare ATCC 49512 312879015 Aminomonas paucivorans DSM 12260 83591793 Rhodospirillum rubrum ATCC 11170 294086111 Candidatus Puniceispirillum marinum IMCC1322 121608211 Verminephrobacter eiseniae EF01-2 344171927 Ralstonia syzygii R24 159042956 Dinoroseobacter shibae DFL 12 288957741 Azospirillum sp-B510 92109262 Nitrobacter hamburgensis X14 148255343 Bradyrhizobium sp-BTAi1 34557790 Wolinella succinogenes DSM 1740 218563121 Campylobacter jejuni subsp. jejuni 291276265 Helicobacter mustelae 12198 229113166 Bacillus cereus Rock1-15 222109285 Acidovorax ebreus TPSY 189485225 uncultured Termite group 1 182624245 Clostridium perfringens D str. 220930482 Clostridium cellulolyticum H10 154250555 Parvibaculum lavamentivorans DS-1 257413184 Roseburia intestinalis L1-82 218767588 Neisseria meningitidis Z2491 15602992 Pasteurella multocida subsp. multocida 319941583 Sutterella wadsworthensis 3 1 254447899 gamma proteobacterium HTCC5015 54296138 Legionella pneumophila str. Paris 331001027 Parasutterella excrementihominis YIT 11859 34557932 Wolinella succinogenes DSM 1740 118497352 Francisella novicida U112 The constructs and methods described herein can include the use of any of those Cas9 proteins, and their corresponding guide RNAs or other guide RNAs that are compatible. The Cas9 from Streptococcus thermophilus LMD-9 CRISPR1 system has also been shown to function in human cells in Cong et al (Science 339, 819 (2013)). Cas9 orthologs from N. meningitides are described in Hou et al., Proc Natl Acad Sci USA. 2013 Sep. 24; 110(39):15644-9 and Esvelt et al., Nat Methods. 2013 November; 10(11):1116-21. Additionally, Jinek et al. showed in vitro that Cas9 orthologs from S. thermophilus and L. innocua, (but not from N. meningitidis or C. jejuni, which likely use a different guide RNA), can be guided by a dual S. pyogenes gRNA to cleave target plasmid DNA, albeit with slightly decreased efficiency.

In some embodiments, the present system utilizes the Cas9 protein from S. pyogenes, either as encoded in bacteria or codon-optimized for expression in mammalian cells, containing mutations at D10, E762, H983, or D986 and H840 or N863, e.g., D10A/D10N and H840A/H840N/H840Y, to render the nuclease portion of the protein catalytically inactive; substitutions at these positions could be alanine (as they are in Nishimasu al., Cell 156, 935-949 (2014)) or they could be other residues, e.g., glutamine, asparagine, tyrosine, serine, or aspartate, e.g., E762Q, H983N, H983Y, D986N, N863D, N863S, or N863H (FIG. 1C). The sequence of the catalytically inactive S. pyogenes Cas9 that can be used in the methods and compositions described herein is as follows; the exemplary mutations of Dl OA and H840A are in bold and underlined.

(SEQ ID NO: 5)         10         20         30         40  MDKKYSIGL A  IGTNSVGWAV ITDEYKVPSK KFKVLGNTDR         50         60         70         80 HSIKKNLIGA LLFDSGETAE ATRLKRTARR RYTRRKNRIC         90        100        110        120 YLQEIFSNEM AKVDDSFFHR LEESFLVEED KKHERHPIFG        130        140        150        160  NIVDEVAYHE KYPTIYHLRK KLVDSTDKAD LRLIYLALAH        170        180        190        200 MIKFRGHFLI EGDLNPDNSD VDKLFIQLVQ TYNQLFEENP        210        220        230        240 INASGVDAKA ILSARLSKSR RLENLIAQLP GEKKNGLFGN        250        260        270        280 LIALSLGLTP NFKSNFDLAE DAKLQLSKDT YDDDLDNLLA        290        300        310        320 QIGDQYADLF LAAKNLSDAI LLSDILRVNT EITKAPLSAS        330        340        350        360 MIKRYDEHHQ DLTLLKALVR QQLPEKYKEI FFDQSKNGYA        370        380        390        400  GYIDGGASQE EFYKFIKPIL EKMDGTEELL VKLNREDLLR        410        420        430        440 KQRTFDNGSI PHQIHLGELH AILRRQEDFY PFLKDNREKI        450        460        470        480 EKILTFRIPY YVGPLARGNS RFAWMTRKSE ETITPWNFEE        490        500        510        520 VVDKGASAQS FIERMTNFDK NLPNEKVLPK HSLLYEYFTV        530        540        550        560 YNELTKVKYV TEGMRKPAFL SGEQKKAIVD LLFKTNRKVT        570        580        590        600 VKQLKEDYFK KIECFDSVEI SGVEDRFNAS LGTYHDLLKI        610        620        630        640         IKDKDFLDNE ENEDILEDIV LTLTLFEDRE MIEERLKTYA        650       660         670        680 HLFDDKVMKQ LKRRRYTGWG RLSRKLINGI RDKQSGKTIL        690        700        710        720 DFLKSDGFAN RNFMQLIHDD SLTFKEDIQK AQVSGQGDSL        730        740        750        760 HEHIANLAGS PAIKKGILQT VKVVDELVKV MGRHKPENIV        770        780        790        800 IEMARENQTT QKGQKNSRER MKRIEEGIKE LGSQILKEHP        810        820        830        840 VENTQLQNEK LYLYYLQNGR DMYVDQELDI NRLSDYDVD A        850        860        870        880 IVPQSFLKDD SIDNKVLTRS DKNRGKSDNV PSEEVVKKMK        890        900        910        920 NYWRQLLNAK LITQRKFDNL TKAERGGLSE LDKAGFIKRQ        930        940        950        960 LVETRQITKH VAQILDSRMN TKYDENDKLI REVKVITLKS        970        980        990       1000 KLVSDFRKDF QFYKVREINN YHHAHDAYLN AVVGTALIKK       1010       1020       1030       1040 YPKLESEFVY GDYKVYDVRK MIAKSEQEIG KATAKYFFYS       1050       1060       1070       1080 NIMNFFKTEI TLANGEIRKR PLIETNGETG EIVWDKGRDF       1090       1100       1110       1120 ATVRKVLSMP QVNIVKKTEV QTGGFSKESI LPKRNSDKLI       1130       1140       1150       1160 ARKKDWDPKK YGGFDSPTVA YSVLVVAKVE KGKSKKLKSV       1170       1180       1190       1200 KELLGITIME RSSFEKNPID FLEAKGYKEV KKDLIIKLPK       1210       1220       1230       1240  YSLFELENGR KRMLASAGEL QKGNELALPS KYVNFLYLAS       1250       1260       1270       1280 HYEKLKGSPE DNEQKQLFVE QHKHYLDEII EQISEFSKRV       1290       1300       1310       1320 ILADANLDKV LSAYNKHRDK PIREQAENII HLFTLTNLGA       1330       1340       1350       1360 PAAFKYFDTT IDRKRYTSTK EVLDATLIHQ SITGLYETRI DLSQLGGD

In some embodiments, the Cas9 nuclease used herein is at least about 50% identical to the sequence of S. pyogenes Cas9, i.e., at least 50% identical to SEQ ID NO:5. In some embodiments, the nucleotide sequences are about 50%, 55%, 60%, 65%, 70%, 75%, 80%, 85%, 90%, 95%, 99% or 100% identical to SEQ ID NO:5. In some embodiments, any differences from SEQ ID NO:5 are in non-conserved regions, as identified by sequence alignment of sequences set forth in Chylinski et al., RNA Biology 10:5, 1-12; 2013 (e.g., in supplementary FIG. 1 and supplementary table 1 thereof); Esvelt et al., Nat Methods. 2013 November; 10(11):1116-21 and Fonfara et al., Nucl. Acids Res. (2014) 42 (4): 2577-2590. [Epub ahead of print 2013 Nov. 22] doi:10.1093/nar/gkt1074. Identity is determined as set forth above.

Guide RNAs (gRNAs)

Guide RNAs generally speaking come in two different systems: System 1, which uses separate crRNA and tracrRNAs that function together to guide cleavage by Cas9, and System 2, which uses a chimeric crRNA-tracrRNA hybrid that combines the two separate guide RNAs in a single system (referred to as a single guide RNA or sgRNA, see also Jinek et al., Science 2012; 337:816-821). The tracrRNA can be variably truncated and a range of lengths has been shown to function in both the separate system (system 1) and the chimeric gRNA system (system 2). For example, in some embodiments, tracrRNA may be truncated from its 3′ end by at least 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 15, 20, 25, 30, 35 or 40 nts. In some embodiments, the tracrRNA molecule may be truncated from its 5′ end by at least 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 15, 20, 25, 30, 35 or 40 nts. Alternatively, the tracrRNA molecule may be truncated from both the 5′ and 3′ end, e.g., by at least 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 15 or 20 nts on the 5′ end and at least 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 15, 20, 25, 30, 35 or 40 nts on the 3′ end. See, e.g., Jinek et al., Science 2012; 337:816-821; Mali et al., Science. 2013 Feb. 15; 339(6121):823-6; Cong et al., Science. 2013 Feb. 15; 339(6121):819-23; and Hwang and Fu et al., Nat Biotechnol. 2013 March; 31(3):227-9; Jinek et al., Elife 2, e00471 (2013)). For System 2, generally the longer length chimeric gRNAs have shown greater on-target activity but the relative specificities of the various length gRNAs currently remain undefined and therefore it may be desirable in certain instances to use shorter gRNAs. In some embodiments, the gRNAs are complementary to a region that is within about 100-800 bp upstream of the transcription start site, e.g., is within about 500 bp upstream of the transcription start site, includes the transcription start site, or within about 100-800 bp, e.g., within about 500 bp, downstream of the transcription start site. In some embodiments, vectors (e.g., plasmids) encoding more than one gRNA are used, e.g., plasmids encoding, 2, 3, 4, 5, or more gRNAs directed to different sites in the same region of the target gene.

Cas9 nuclease can be guided to specific 17-20 nt genomic targets bearing an additional proximal protospacer adjacent motif (PAM), e.g., of sequence NGG, using a guide RNA, e.g., a single gRNA or a tracrRNA/crRNA, bearing 17-20 nts at its 5′ end that are complementary to the complementary strand of the genomic DNA target site. Thus, the present methods can include the use of a single guide RNA comprising a crRNA fused to a normally trans-encoded tracrRNA, e.g., a single Cas9 guide RNA as described in Mali et al., Science 2013 Feb. 15; 339(6121):823-6, with a sequence at the 5′ end that is complementary to the target sequence, e.g., of 25-17, optionally 20 or fewer nucleotides (nts), e.g., 20, 19, 18, or 17 nts, preferably 17 or 18 nts, of the complementary strand to a target sequence immediately 5′ of a protospacer adjacent motif (PAM), e.g., NGG, NAG, or NNGG. In some embodiments, the single Cas9 guide RNA consists of the sequence:

(X₁₇₋₂₀)GUUUUAGAGCUAGAAAUAGCAAGUUAAAAUAAGGCUAGUC CG(X_(N)) (SEQ ID NO:6);

(X₁₇₋₂₀)GUUUUAGAGCUAUGCUGAAAAGCAUAGCAAGUUAAAAUAAGGCU AGUCCGUUAUC(X_(N)) (SEQ ID NO:7);

(X₁₇₋₂₀)GUUUUAGAGCUAUGCUGUUUUGGAAACAAAACAGCAUAGCAAGU UAAAAUAAGGCUAGUCCGUUAUC(X_(N)) (SEQ ID NO:8);

(X₁₇₋₂₀)GUUUUAGAGCUAGAAAUAGCAAGUUAAAAUAAGGCUAGUCCGUU AUCAACUUGAAAAAGUGGCACCGAGUCGGUGC(X_(N)) (SEQ ID NO:9),

(X₁₇₋₂₀)GUUUAAGAGCUAGAAAUAGCAAGUUUAAAUAAGGCUAGUCCGUU AUCAACUUGAAAAAGUGGCACCGAGUCGGUGC(SEQ ID NO:10);

(X₁₇₋₂₀)GUUUUAGAGCUAUGCUGGAAACAGCAUAGCAAGUUUAAAUAAGG CUAGUCCGUUAUCAACUUGAAAAAGUGGCACCGAGUCGGUGC (SEQ ID NO:11); or

(X₁₇₋₂₀)GUUUAAGAGCUAUGCUGGAAACAGCAUAGCAAGUUUAAAUAAGG CUAGUCCGUUAUCAACUUGAAAAAGUGGCACCGAGUCGGUGC (SEQ ID NO:12);

wherein X₁₇₋₂₀ is the nucleotide sequence complementary to 17-20 consecutive nucleotides of the target sequence. DNAs encoding the single guide RNAs have been described previously in the literature (Jinek et al., Science. 337(6096):816-21 (2012) and Jinek et al., Elife. 2:e00471 (2013)).

The guide RNAs can include X_(N) which can be any sequence, wherein N (in the RNA) can be 0-200, e.g., 0-100, 0-50, or 0-20, that does not interfere with the binding of the ribonucleic acid to Cas9.

In some embodiments, the guide RNA includes one or more Adenine (A) or Uracil (U) nucleotides on the 3′ end. In some embodiments the RNA includes one or more U, e.g., 1 to 8 or more Us (e.g., U, UU, UUU, UUUU, UUUUU, UUUUUU, UUUUUUU, UUUUUUUU) at the 3′ end of the molecule, as a result of the optional presence of one or more Ts used as a termination signal to terminate RNA PolIII transcription.

Although some of the examples described herein utilize a single gRNA, the methods can also be used with dual gRNAs (e.g., the crRNA and tracrRNA found in naturally occurring systems). In this case, a single tracrRNA would be used in conjunction with multiple different crRNAs expressed using the present system, e.g., the following:

(X₁₇₋₂₀)GUUUUAGAGCUA (SEQ ID NO:13);

(X₁₇₋₂₀)GUUUUAGAGCUAUGCUGUUUUG (SEQ ID NO:14); or

(X₁₇₋₂₀)GUUUUAGAGCUAUGCU (SEQ ID NO:15); and a tracrRNA sequence. In this case, the crRNA is used as the guide RNA in the methods and molecules described herein, and the tracrRNA can be expressed from the same or a different DNA molecule. In some embodiments, the methods include contacting the cell with a tracrRNA comprising or consisting of the sequence GGAACCAUUCAAAACAGCAUAGCAAGUUAAAAUAAGGCUAGUCCGUUA UCAACUUGAAAAAGUGGCACCGAGUCGGUGC (SEQ ID NO:16) or an active portion thereof (an active portion is one that retains the ability to form complexes with Cas9 or dCas9). In some embodiments, the tracrRNA molecule may be truncated from its 3′ end by at least 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 15, 20, 25, 30, 35 or 40 nts. In another embodiment, the tracrRNA molecule may be truncated from its 5′ end by at least 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 15, 20, 25, 30, 35 or 40 nts. Alternatively, the tracrRNA molecule may be truncated from both the 5′ and 3′ end, e.g., by at least 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 15 or 20 nts on the 5′ end and at least 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 15, 20, 25, 30, 35 or 40 nts on the 3′ end. Exemplary tracrRNA sequences in addition to SEQ ID NO:8 include the following: UAGCAAGUUAAAAUAAGGCUAGUCCGUUAUCAACUUGAAAAAGUGGCA CCGAGUCGGUGC (SEQ ID NO:17) or an active portion thereof; or AGCAUAGCAAGUUAAAAUAAGGCUAGUCCGUUAUCAACUUGAAAAAGU GGCACCGAGUCGGUGC (SEQ ID NO:18) or an active portion thereof.

In some embodiments wherein (X₁₇₋₂₀)GUUUUAGAGCUAUGCUGUUUUG (SEQ ID NO:14) is used as a crRNA, the following tracrRNA is used:

GGAACCAUUCAAAACAGCAUAGCAAGUUAAAAUAAGGCUAGUCCGUUA UCAACUUGAAAAAGUGGCACCGAGUCGGUGC (SEQ ID NO:16) or an active portion thereof. In some embodiments wherein (X₁₇₋₂₀)GUUUUAGAGCUA (SEQ ID NO:13) is used as a crRNA, the following tracrRNA is used:

UAGCAAGUUAAAAUAAGGCUAGUCCGUUAUCAACUUGAAAAAGUGGCA CCGAGUCGGUGC (SEQ ID NO:17) or an active portion thereof. In some embodiments wherein (X₁₇₋₂₀)GUUUUAGAGCUAUGCU (SEQ ID NO:15) is used as a crRNA, the following tracrRNA is used:

AGCAUAGCAAGUUAAAAUAAGGCUAGUCCGUUAUCAACUUGAAAAAGU GGCACCGAGUCGGUGC (SEQ ID NO:18) or an active portion thereof.

In some embodiments, the gRNA is targeted to a site that is at least three or more mismatches different from any sequence in the rest of the genome in order to minimize off-target effects.

Modified RNA oligonucleotides such as locked nucleic acids (LNAs) have been demonstrated to increase the specificity of RNA-DNA hybridization by locking the modified oligonucleotides in a more favorable (stable) conformation. For example, 2′-O-methyl RNA is a modified base where there is an additional covalent linkage between the 2′ oxygen and 4′ carbon which when incorporated into oligonucleotides can improve overall thermal stability and selectivity (Formula I).

Thus in some embodiments, the tru-gRNAs disclosed herein may comprise one or more modified RNA oligonucleotides. For example, the truncated guide RNAs molecules described herein can have one, some or all of the 17-18 or 17-19 nts 5′ region of the guideRNA complementary to the target sequence are modified, e.g., locked (2′-O-4′-C methylene bridge), 5′-methylcytidine, 2′-O-methyl-pseudouridine, or in which the ribose phosphate backbone has been replaced by a polyamide chain (peptide nucleic acid), e.g., a synthetic ribonucleic acid.

In other embodiments, one, some or all of the nucleotides of the tru-gRNA sequence may be modified, e.g., locked (2′-O-4′-C methylene bridge), 5′-methylcytidine, 2′-O-methyl-pseudouridine, or in which the ribose phosphate backbone has been replaced by a polyamide chain (peptide nucleic acid), e.g., a synthetic ribonucleic acid.

In some embodiments, the single guide RNAs and/or crRNAs and/or tracrRNAs can include one or more Adenine (A) or Uracil (U) nucleotides on the 3′ end.

Existing Cas9-based RGNs use gRNA-DNA heteroduplex formation to guide targeting to genomic sites of interest. However, RNA-DNA heteroduplexes can form a more promiscuous range of structures than their DNA-DNA counterparts. In effect, DNA-DNA duplexes are more sensitive to mismatches, suggesting that a DNA-guided nuclease may not bind as readily to off-target sequences, making them comparatively more specific than RNA-guided nucleases. Thus, the guide RNAs usable in the methods described herein can be hybrids, i.e., wherein one or more deoxyribonucleotides, e.g., a short DNA oligonucleotide, replaces all or part of the gRNA, e.g., all or part of the complementarity region of a gRNA. This DNA-based molecule could replace either all or part of the gRNA in a single gRNA system or alternatively might replace all of part of the crRNA and/or tracrRNA in a dual crRNA/tracrRNA system. Such a system that incorporates DNA into the complementarity region should more reliably target the intended genomic DNA sequences due to the general intolerance of DNA-DNA duplexes to mismatching compared to RNA-DNA duplexes. Methods for making such duplexes are known in the art, See, e.g., Barker et al., BMC Genomics. 2005 Apr. 22; 6:57; and Sugimoto et al., Biochemistry. 2000 Sep. 19; 39(37):11270-81.

In addition, in a system that uses separate crRNA and tracrRNA, one or both can be synthetic and include one or more modified (e.g., locked) nucleotides or deoxyribonucleotides.

In a cellular context, complexes of Cas9 with these synthetic gRNAs could be used to improve the genome-wide specificity of the CRISPR/Cas9 nuclease system.

The methods described can include expressing in a cell, or contacting the cell with, a Cas9 gRNA plus a fusion protein as described herein.

Expression Systems

In order to use the fusion proteins described, it may be desirable to express them from a nucleic acid that encodes them. This can be performed in a variety of ways. For example, the nucleic acid encoding the guide RNA can be cloned into an intermediate vector for transformation into prokaryotic or eukaryotic cells for replication and/or expression. Intermediate vectors are typically prokaryote vectors, e.g., plasmids, or shuttle vectors, or insect vectors, for storage or manipulation of the nucleic acid encoding the fusion proteins for production of the fusion proteins. The nucleic acid encoding the fusion proteins can also be cloned into an expression vector, for administration to a plant cell, animal cell, preferably a mammalian cell or a human cell, fungal cell, bacterial cell, or protozoan cell.

To obtain expression, a sequence encoding a fusion protein is typically subcloned into an expression vector that contains a promoter to direct transcription. Suitable bacterial and eukaryotic promoters are well known in the art and described, e.g., in Sambrook et al., Molecular Cloning, A Laboratory Manual (3d ed. 2001); Kriegler, Gene Transfer and Expression: A Laboratory Manual (1990); and Current Protocols in Molecular Biology (Ausubel et al., eds., 2010). Bacterial expression systems for expressing the engineered protein are available in, e.g., E. coli, Bacillus sp., and Salmonella (Palva et al., 1983, Gene 22:229-235). Kits for such expression systems are commercially available. Eukaryotic expression systems for mammalian cells, yeast, and insect cells are well known in the art and are also commercially available.

The promoter used to direct expression of a nucleic acid depends on the particular application. For example, a strong constitutive promoter is typically used for expression and purification of fusion proteins. In contrast, when the guide RNA is to be administered in vivo for gene regulation, either a constitutive or an inducible promoter can be used, depending on the particular use of the guide RNA. In addition, a preferred promoter for administration of the guide RNA can be a weak promoter, such as HSV TK or a promoter having similar activity. The promoter can also include elements that are responsive to transactivation, e.g., hypoxia response elements, Ga14 response elements, lac repressor response element, and small molecule control systems such as tetracycline-regulated systems and the RU-486 system (see, e.g., Gossen & Bujard, 1992, Proc. Natl. Acad. Sci. USA, 89:5547; Oligino et al., 1998, Gene Ther., 5:491-496; Wang et al., 1997, Gene Ther., 4:432-441; Neering et al., 1996, Blood, 88:1147-55; and Rendahl et al., 1998, Nat. Biotechnol., 16:757-761).

In addition to the promoter, the expression vector typically contains a transcription unit or expression cassette that contains all the additional elements required for the expression of the nucleic acid in host cells, either prokaryotic or eukaryotic. A typical expression cassette thus contains a promoter operably linked, e.g., to the nucleic acid sequence encoding the gRNA, and any signals required, e.g., for efficient polyadenylation of the transcript, transcriptional termination, ribosome binding sites, or translation termination. Additional elements of the cassette may include, e.g., enhancers, and heterologous spliced intronic signals.

The particular expression vector used to transport the genetic information into the cell is selected with regard to the intended use of the gRNA, e.g., expression in plants, animals, bacteria, fungus, protozoa, etc. Standard bacterial expression vectors include plasmids such as pBR322 based plasmids, pSKF, pET23D, and commercially available tag-fusion expression systems such as GST and LacZ.

Expression vectors containing regulatory elements from eukaryotic viruses are often used in eukaryotic expression vectors, e.g., SV40 vectors, papilloma virus vectors, and vectors derived from Epstein-Barr virus. Other exemplary eukaryotic vectors include PMSG, pAV009/A+, pMTO10/A+, pMAMneo-5, baculovirus pDSVE, and any other vector allowing expression of proteins under the direction of the SV40 early promoter, SV40 late promoter, metallothionein promoter, murine mammary tumor virus promoter, Rous sarcoma virus promoter, polyhedrin promoter, or other promoters shown effective for expression in eukaryotic cells.

The vectors for expressing the guide RNAs can include RNA Pol III promoters to drive expression of the guide RNAs, e.g., the H1, U6 or 7SK promoters. These human promoters allow for expression of gRNAs in mammalian cells following plasmid transfection. Alternatively, a T7 promoter may be used, e.g., for in vitro transcription, and the RNA can be transcribed in vitro and purified. Vectors suitable for the expression of short RNAs, e.g., siRNAs, shRNAs, or other small RNAs, can be used. With the Cys4-based multiplex system described in FIG. 4B, multiple gRNAs can be expressed in a single transcript (driven by a RNA Pol II or Pol III promoter) and then cleaved out from that larger transcript.

Some expression systems have markers for selection of stably transfected cell lines such as thymidine kinase, hygromycin B phosphotransferase, and dihydrofolate reductase. High yield expression systems are also suitable, such as using a baculovirus vector in insect cells, with the gRNA encoding sequence under the direction of the polyhedrin promoter or other strong baculovirus promoters.

The elements that are typically included in expression vectors also include a replicon that functions in E. coli, a gene encoding antibiotic resistance to permit selection of bacteria that harbor recombinant plasmids, and unique restriction sites in nonessential regions of the plasmid to allow insertion of recombinant sequences.

Standard transfection methods are used to produce bacterial, mammalian, yeast or insect cell lines that express large quantities of protein, which are then purified using standard techniques (see, e.g., Colley et al., 1989, J. Biol. Chem., 264:17619-22; Guide to Protein Purification, in Methods in Enzymology, vol. 182 (Deutscher, ed., 1990)). Transformation of eukaryotic and prokaryotic cells are performed according to standard techniques (see, e.g., Morrison, 1977, J. Bacteriol. 132:349-351; Clark-Curtiss & Curtiss, Methods in Enzymology 101:347-362 (Wu et al., eds, 1983).

Any of the known procedures for introducing foreign nucleotide sequences into host cells may be used. These include the use of calcium phosphate transfection, polybrene, protoplast fusion, electroporation, nucleofection, liposomes, microinjection, naked DNA, plasmid vectors, viral vectors, both episomal and integrative, and any of the other well-known methods for introducing cloned genomic DNA, cDNA, synthetic DNA or other foreign genetic material into a host cell (see, e.g., Sambrook et al., supra). It is only necessary that the particular genetic engineering procedure used be capable of successfully introducing at least one gene into the host cell capable of expressing the gRNA.

The present invention includes the vectors and cells comprising the vectors.

EXAMPLES

The invention is further described in the following examples, which do not limit the scope of the invention described in the claims.

Example 1. Assessing Specificity of RNA-Guided Endonucleases

CRISPR RNA-guided nucleases (RGNs) have rapidly emerged as a facile and efficient platform for genome editing. This example describes the use of a human cell-based reporter assay to characterize off-target cleavage of Cas9-based RGNs.

Materials and Methods

The following materials and methods were used in Example 1.

Construction of Guide RNAs

DNA oligonucleotides harboring variable 20 nt sequences for Cas9 targeting were annealed to generate short double-strand DNA fragments with 4 bp overhangs compatible with ligation into BsmBI-digested plasmid pMLM3636. Cloning of these annealed oligonucleotides generates plasmids encoding a chimeric+103 single-chain guide RNA with 20 variable 5′ nucleotides under expression of a U6 promoter (Hwang et al., Nat Biotechnol 31, 227-229 (2013); Mali et al., Science 339, 823-826 (2013)). pMLM3636 and the expression plasmid pJDS246 (encoding a codon optimized version of Cas9) used in this study are both available through the non-profit plasmid distribution service Addgene (addgene.org/crispr-cas).

EGFP Activity Assays

U2OS.EGFP cells harboring a single integrated copy of an EGFP-PEST fusion gene were cultured as previously described (Reyon et al., Nat Biotech 30, 460-465 (2012)). For transfections, 200,000 cells were Nucleofected with the indicated amounts of gRNA expression plasmid and pJDS246 together with 30 ng of a Td-tomato-encoding plasmid using the SE Cell Line 4D-Nucleofector™ X Kit (Lonza) according to the manufacturer's protocol. Cells were analyzed 2 days post-transfection using a BD LSRII flow cytometer. Transfections for optimizing gRNA/Cas9 plasmid concentration were performed in triplicate and all other transfections were performed in duplicate.

PCR Amplification and Sequence Verification of Endogenous Human Genomic Sites

PCR reactions were performed using Phusion Hot Start II high-fidelity DNA polymerase (NEB). Most loci amplified successfully using touchdown PCR (98° C., 10 s; 72-62° C., −1° C./cycle, 15 s; 72° C., 30 s] 10 cycles, [98° C., 10 s; 62° C., 15 s; 72° C., 30 s] 25 cycles). PCR for the remaining targets were performed with 35 cycles at a constant annealing temperature of 68° C. or 72° C. and 3% DMSO or 1M betaine, if necessary. PCR products were analyzed on a QIAXCEL capillary electrophoresis system to verify both size and purity. Validated products were treated with ExoSap-IT (Affymetrix) and sequenced by the Sanger method (MGH DNA Sequencing Core) to verify each target site.

Determination of RGN-Induced on- and Off-Target Mutation Frequencies in Human Cells

For U2OS.EGFP and K562 cells, 2×10⁵ cells were transfected with 250 ng of gRNA expression plasmid or an empty U6 promoter plasmid (for negative controls), 750 ng of Cas9 expression plasmid, and 30 ng of td-Tomato expression plasmid using the 4D Nucleofector System according to the manufacturer's instructions (Lonza). For HEK293 cells, 1.65×10⁵ cells were transfected with 125 ng of gRNA expression plasmid or an empty U6 promoter plasmid (for the negative control), 375 ng of Cas9 expression plasmid, and 30 ng of a td-Tomato expression plasmid using Lipofectamine LTX reagent according to the manufacturer's instructions (Life Technologies). Genomic DNA was harvested from transfected U2OS.EGFP, HEK293, or K562 cells using the QIAamp DNA Blood Mini Kit (QIAGEN), according to the manufacturer's instructions. To generate enough genomic DNA to amplify the off-target candidate sites, DNA from three Nucleofections (for U2OS.EGFP cells), two Nucleofections (for K562 cells), or two Lipofectamine LTX transfections was pooled together before performing T7EI. This was done twice for each condition tested, thereby generating duplicate pools of genomic DNA representing a total of four or six individual transfections. PCR was then performed using these genomic DNAs as templates as described above and purified using Ampure XP beads (Agencourt) according to the manufacturer's instructions. T7EI assays were performed as previously described (Reyon et al., 2012, supra).

DNA Sequencing of NHEJ-Mediated Indel Mutations

Purified PCR products used for the T7EI assay were cloned into Zero Blunt TOPO vector (Life Technologies) and plasmid DNAs were isolated using an alkaline lysis miniprep method by the MGH DNA Automation Core. Plasmids were sequenced using an M13 forward primer (5′-GTAAAACGACGGCCAG-3′ (SEQ ID NO:19)) by the Sanger method (MGH DNA Sequencing Core).

Example 1a. Single Nucleotide Mismatches

To begin to define the specificity determinants of RGNs in human cells, a large-scale test was performed to assess the effects of systematically mismatching various positions within multiple gRNA/target DNA interfaces. To do this, a quantitative human cell-based enhanced green fluorescent protein (EGFP) disruption assay previously described (see Methods above and Reyon et al., 2012, supra) that enables rapid quantitation of targeted nuclease activities (FIG. 2B) was used. In this assay, the activities of nucleases targeted to a single integrated EGFP reporter gene can be quantified by assessing loss of fluorescence signal in human U2OS.EGFP cells caused by inactivating frameshift insertion/deletion (indel) mutations introduced by error prone non-homologous end joining (NHEJ) repair of nuclease-induced double-stranded breaks (DSBs) (FIG. 2B). For the studies described here, three ˜100 nt single gRNAs (sgRNAs) targeted to different sequences within EGFP were used, as follows:

EGFP Site 1 (SEQ ID NO: 1) GGGCACGGGCAGCTTGCCGGTGG  EGFP Site 2 (SEQ ID NO: 2) GATGCCGTTCTTCTGCTTGTCGG  EGFP Site 3 (SEQ ID NO: 3) GGTGGTGCAGATGAACTTCAGGG  Each of these sgRNAs can efficiently direct Cas9-mediated disruption of EGFP expression (see Example 1e and 2a, and FIGS. 3E (top) and 3F (top)).

In initial experiments, the effects of single nucleotide mismatches at 19 of 20 nucleotides in the complementary targeting region of three EGFP-targeted sgRNAs were tested. To do this, variant sgRNAs were generated for each of the three target sites harboring Watson-Crick transversion mismatches at positions 1 through 19 (numbered 1 to 20 in the 3′ to 5′ direction; see FIG. 1) and the abilities of these various sgRNAs to direct Cas9-mediated EGFP disruption in human cells tested (variant sgRNAs bearing a substitution at position 20 were not generated because this nucleotide is part of the U6 promoter sequence and therefore must remain a guanine to avoid affecting expression.)

For EGFP target site #2, single mismatches in positions 1-10 of the gRNA have dramatic effects on associated Cas9 activity (FIG. 2C, middle panel), consistent with previous studies that suggest mismatches at the 5′ end of gRNAs are better tolerated than those at the 3′ end (Jiang et al., Nat Biotechnol 31, 233-239 (2013); Cong et al., Science 339, 819-823 (2013); Jinek et al., Science 337, 816-821 (2012)). However, with EGFP target sites #1 and #3, single mismatches at all but a few positions in the gRNA appear to be well tolerated, even within the 3′ end of the sequence. Furthermore, the specific positions that were sensitive to mismatch differed for these two targets (FIG. 2C, compare top and bottom panels)—for example, target site #1 was particularly sensitive to a mismatch at position 2 whereas target site #3 was most sensitive to mismatches at positions 1 and 8.

Example 1b. Multiple Mismatches

To test the effects of more than one mismatch at the gRNA/DNA interface, a series of variant sgRNAs bearing double Watson-Crick transversion mismatches in adjacent and separated positions were created and the abilities of these sgRNAs to direct Cas9 nuclease activity were tested in human cells using the EGFP disruption assay. All three target sites generally showed greater sensitivity to double alterations in which one or both mismatches occur within the 3′ half of the gRNA targeting region. However, the magnitude of these effects exhibited site-specific variation, with target site #2 showing the greatest sensitivity to these double mismatches and target site #1 generally showing the least. To test the number of adjacent mismatches that can be tolerated, variant sgRNAs were constructed bearing increasing numbers of mismatched positions ranging from positions 19 to 15 in the 5′ end of the gRNA targeting region (where single and double mismatches appeared to be better tolerated).

Testing of these increasingly mismatched sgRNAs revealed that for all three target sites, the introduction of three or more adjacent mismatches results in significant loss of RGN activity. A sudden drop off in activity occurred for three different EGFP-targeted gRNAs as one makes progressive mismatches starting from position 19 in the 5′ end and adding more mismatches moving toward the 3′ end. Specifically, gRNAs containing mismatches at positions 19 and 19+18 show essentially full activity whereas those with mismatches at positions 19+18+17, 19+18+17+16, and 19+18+17+16+15 show essentially no difference relative to a negative control (FIG. 2F). (Note that we did not mismatch position 20 in these variant gRNAs because this position needs to remain as a G because it is part of the U6 promoter that drives expression of the gRNA.)

Additional proof of that shortening gRNA complementarity might lead to RGNs with greater specificities was obtained in the following experiment: for four different EGFP-targeted gRNAs (FIG. 2H), introduction of a double mismatch at positions 18 and 19 did not significantly impact activity. However, introduction of another double mismatch at positions 10 and 11 then into these gRNAs results in near complete loss of activity. Interestingly introduction of only the 10/11 double mismatches does not generally have as great an impact on activity.

Taken together, these results in human cells confirm that the activities of RGNs can be more sensitive to mismatches in the 3′ half of the gRNA targeting sequence. However, the data also clearly reveal that the specificity of RGNs is complex and target site-dependent, with single and double mismatches often well tolerated even when one or more mismatches occur in the 3′ half of the gRNA targeting region. Furthermore, these data also suggest that not all mismatches in the 5′ half of the gRNA/DNA interface are necessarily well tolerated.

In addition, these results strongly suggest that gRNAs bearing shorter regions of complementarity (specifically ˜17 nts) will be more specific in their activities. We note that 17 nts of specificity combined with the 2 nts of specificity conferred by the PAM sequence results in specification of a 19 bp sequence, one of sufficient length to be unique in large complex genomes such as those found in human cells.

Example 1c. Off-Target Mutations

To determine whether off-target mutations for RGNs targeted to endogenous human genes could be identified, six sgRNAs that target three different sites in the VEGFA gene, one in the EMX1 gene, one in the RNF2 gene, and one in the FANCF gene were used. These six sgRNAs efficiently directed Cas9-mediated indels at their respective endogenous loci in human U2OS.EGFP cells as detected by T7 Endonuclease I (T7EI) assay (Methods above). For each of these six RGNs, we then examined dozens of potential off-target sites (ranging in number from 46 to as many as 64) for evidence of nuclease-induced NHEJ-mediated indel mutations in U2OS.EGFP cells. The loci assessed included all genomic sites that differ by one or two nucleotides as well as subsets of genomic sites that differ by three to six nucleotides and with a bias toward those that had one or more of these mismatches in the 5′ half of the gRNA targeting sequence. Using the T7EI assay, four off-target sites (out of 53 candidate sites examined) for VEGFA site 1, twelve (out of 46 examined) for VEGFA site 2, seven (out of 64 examined) for VEGFA site 3 and one (out of 46 examined) for the EMX1 site were readily identified. No off-target mutations were detected among the 43 and 50 potential sites examined for the RNF2 or FANCF genes, respectively. The rates of mutation at verified off-target sites were very high, ranging from 5.6% to 125% (mean of 40%) of the rate observed at the intended target site. These bona fide off-targets included sequences with mismatches in the 3′ end of the target site and with as many as a total of five mismatches, with most off-target sites occurring within protein coding genes. DNA sequencing of a subset of off-target sites provided additional molecular confirmation that indel mutations occur at the expected RGN cleavage site (FIGS. 8A-C).

Example 1d. Off-Target Mutations in Other Cell Types

Having established that RGNs can induce off-target mutations with high frequencies in U2OS.EGFP cells, it was next sought to determine whether these nucleases would also have these effects in other types of human cells. U2OS.EGFP cells had been chosen for initial experiments because these cells were previously used to evaluate the activities of TALENs¹⁵ but human HEK293 and K562 cells have been more widely used to test the activities of targeted nucleases. Therefore, the activities of the four RGNs targeted to VEGFA sites 1, 2, and 3 and the EMX1 site were also assessed in HEK293 and K562 cells. Each of these four RGNs efficiently induced NHEJ-mediated indel mutations at their intended on-target site in these two additional human cell lines (as assessed by T7EI assay), albeit with somewhat lower mutation frequencies than those observed in U2OS.EGFP cells. Assessment of the 24 off-target sites for these four RGNs originally identified in U2OS.EGFP cells revealed that many were again mutated in HEK293 and K562 cells with frequencies similar to those at their corresponding on-target site. As expected, DNA sequencing of a subset of these off-target sites from HEK293 cells provided additional molecular evidence that alterations are occurring at the expected genomic loci. It is not known for certain why in HEK293 cells four and in K562 cells eleven of the off-target sites identified in U2OS.EGFP cells did not show detectable mutations. However, many of these off-target sites also showed relatively lower mutation frequencies in U2OS.EGFP cells. Therefore, mutation rates of these sites in HEK293 and K562 cells may be falling below the reliable detection limit of our T7EI assay (˜2-5%) because RGNs generally appear to have lower activities in HEK293 and K562 cells compared with U2OS.EGFP cells in our experiments. Taken together, the results in HEK293 and K562 cells provide evidence that the high-frequency off-target mutations we observe with RGNs will be a general phenomenon seen in multiple human cell types.

Example 1e. Titration of gRNA- and Cas9-Expressing Plasmid Amounts Used for the EGFP Disruption Assay

Single guide RNAs (sgRNAs) were generated for three different sequences (EGFP SITES 1-3, shown above) located upstream of EGFP nucleotide 502, a position at which the introduction of frameshift mutations via non-homologous end-joining can robustly disrupt expression of EGFP (Maeder, M. L. et al., Mol Cell 31, 294-301 (2008); Reyon, D. et al., Nat Biotech 30, 460-465 (2012)).

For each of the three target sites, a range of gRNA-expressing plasmid amounts (12.5 to 250 ng) was initially transfected together with 750 ng of a plasmid expressing a codon-optimized version of the Cas9 nuclease into our U2OS.EGFP reporter cells bearing a single copy, constitutively expressed EGFP-PEST reporter gene. All three RGNs efficiently disrupted EGFP expression at the highest concentration of gRNA plasmid (250 ng) (FIG. 3E (top)). However, RGNs for target sites #1 and #3 exhibited equivalent levels of disruption when lower amounts of gRNA-expressing plasmid were transfected whereas RGN activity at target site #2 dropped immediately when the amount of gRNA-expressing plasmid transfected was decreased (FIG. 3E (top)).

The amount of Cas9-encoding plasmid (range from 50 ng to 750 ng) transfected into our U2OS.EGFP reporter cells was titrated EGFP disruption assayed. As shown in FIG. 3F (top), target site #1 tolerated a three-fold decrease in the amount of Cas9-encoding plasmid transfected without substantial loss of EGFP disruption activity. However, the activities of RGNs targeting target sites #2 and #3 decreased immediately with a three-fold reduction in the amount of Cas9 plasmid transfected (FIG. 3F (top)). Based on these results, 25 ng/250 ng, 250 ng/750 ng, and 200 ng/750 ng of gRNA-/Cas9-expressing plasmids were used for EGFP target sites #1, #2, and #3, respectively, for the experiments described in Examples 1a-1d.

The reasons why some gRNA/Cas9 combinations work better than others in disrupting EGFP expression is not understood, nor is why some of these combinations are more or less sensitive to the amount of plasmids used for transfection. Although it is possible that the range of off-target sites present in the genome for these three sgRNAs might influence each of their activities, no differences were seen in the numbers of genomic sites that differ by one to six bps for each of these particular target sites (Table 1) that would account for the differential behavior of the three sgRNAs.

TABLE 1 Numbers of off-target sites in the human genome for six RGNs targeted to endogenous human genes and three RGNs targeted to the EGFP reporter gene Number of mismatches to on-target site Target Site 0 1 2 3 4 5 6 Target 1 (VEGFA Site 1) 1 1 4 32 280 2175 13873 Target 2 (VEGFA Site 2) 1 0 2 35 443 3889 17398 Target 3 (VEGFA Site 3) 1 1 17 377 6028 13398 35517 Target 4 (EMX) 1 0 1 18 276 2309 15731 Target 5 (RNF2) 1 0 0 6 116 976 7443 Target 6 (FANCF) 1 0 1 18 271 1467 9551 EGFP Target Site #1 0 0 3 10 156 1365 9755 EGFP Target Site #2 0 0 0 11 96 974 7353 EGFP Target Site #3 0 0 1 14 165 1439 10361 Off-target sites for each of the six RGNs targeted to the VEGFA, RNF2, FANCF, and EMX1 genes and the three RGNs targeted to EGFP Target Sites #1, #2 and #3 were identified in human genome sequence build GRCh37. Mismatches were only allowed for the 20 nt region to which the gRNA anneals and not to the PAM sequence.

Example 2. Using Pairs of GuideRNAs with FokI-dCas9 Fusion Proteins

Monomeric CRISPR-Cas9 nucleases are widely used for targeted genome editing but can induce unwanted off-target mutations with high frequencies. This example describes new dimeric RNA-guided FokI Nucleases (RFNs) that recognize an extended, double-length sequence and that strictly depend on two single guide RNAs (gRNAs) for cleavage activity. RFNs can robustly edit DNA sequences in endogenous human genes with high efficiencies. Additionally, a method for expressing gRNAs bearing any 5′ end nucleotide is described, a critical advance that gives dimeric RFNs a useful targeting range. In direct comparisons, monomeric Cas9 nickases generally induce unwanted indels and unexpected focal point mutations with higher frequencies than RFNs directed by a matched single gRNA. RFNs combine the ease of CRISPR RNA-based targeting with the specificity enhancements of dimerization and provide an important new platform for research and therapeutic applications that require highly precise genome editing.

Materials and Methods

The following materials and methods were used in Example 2.

Single and Multiplex gRNA Expression Plasmids

Plasmids encoding single or multiplex gRNAs were assembled in a single-step ligation of annealed target site oligosduplexes (Integrated DNA Technologies) and a constant region oligoduplex (for multiplex gRNAs) with BsmBI-digested Csy4-flanked gRNA backbone (pSQT1313; Addgene).

Multiplex gRNA encoding plasmids were constructed by ligating: 1) annealed oligos encoding the first target site, 2) phosphorylated annealed oligos encoding crRNA, tracrRNA, and Csy4-binding site, and 3) annealed oligos encoding the second target site, into a U6-Csy4site-gRNA plasmid backbone digested with BsmBI Type IIs restriction enzyme. Csy4 RNA binding sites were attached to the 3′ and 5′ ends of a gRNA sequence and expressed with Cas9 in cells. The Csy4 RNA binding site sequence ‘GUUCACUGCCGUAUAGGCAGCUAAGAAA’ (SEQ ID NO:20) was fused to the 5′ and 3′ end of the standard gRNA sequence.

(SEQ ID NO: 21) GUUCACUGCCGUAUAGGCAGNNNNNNNNNNNNNNNNNNNN GUUUUAGAGCUAGAAAUAGCAAGUUAAAAUAAGGCUAGUC CGUUAUCAACUUGAAAAAGUGGCACCGAGUCGGUGCGUUC ACUGCCGUAUAGGCAGNNNNNNNNNNNNNNNNNNNNGUUU UAGAGCUAGAAAUAGCAAGUUAAAAUAAGGCUAGUCCGUU AUCAACUUGAAAAAGUGGCACCGAGUCGGUGCGUUCACUG CCGUAUAGGCAG This sequence is a multiplex gRNA sequence flanked by Csy4 sites (underlined). Functionally, encoding these in multiplex on one transcript should have the same result as encoding them separately. Although all pairs of Csy4-flanked sgRNAs were expressed in a multiplex context in the experiments described herein, the sgRNAs can be encoded in multiplex sgRNAs separated by Csy4 sites encoded on one transcript as well as individual sgRNAs that have an additional Csy4 sequence. In this sequence, the first N20 sequence represents the sequence complementary to one strand of the target genomic sequence, and the second N20 sequence represents the sequence complementary to the other strand of the target genomic sequence.

A plasmid encoding the Csy4 recognition site containing gRNA was co-transfected with plasmid encoding Cas9 and Csy4 proteins separated by a ‘2A’ peptide linkage. The results showed that gRNAs with Csy4 sites fused to the 5′ and 3′ ends remained capable of directing Cas9-mediated cleavage in human cells using the U2OS-EGFP disruption assay previously described. Thus, Csy4 RNA binding sites can be attached to 3′ end of a gRNA sequence and complexes of these Csy4 site-containing gRNAs with Cas9 remain functional in the cell.

In some experiments, a construct encoding Csy4-T2A-FokI-dCas9 was used. The sequences of the FokI-dCas9 fusions are shown below, and include a GGGGS (SEQ ID NO:23) linker (underlined) between the FokI and dCas9 and a nuclear localization sequence.

FokI-dCas9 amino acid sequence (FokI-G4S-dCas9- nls-3XFLAG) (SEQ ID NO: 24) MQLVKSELEEKKSELRHKLKYVPHEYIELIEIARNSTQDRILEMKVMEFF MKVYGYRGKHLGGSRKPDGAIYTVGSPIDYGVIVDTKAYSGGYNLPIGQA DEMQRYVEENQTRNKHINPNEWWKVYPSSVTEFKFLFVSGHFKGNYKAQL TRLNHITNCNGAVLSVEELLIGGEMIKAGTLTLEEVRRKFNNGEINFGGG GSDKKYSIGLAIGTNSVGWAVITDEYKVPSKKFKVLGNTDRHSIKKNLIG ALLFDSGETAEATRLKRTARRRYTRRKNRICYLQEIFSNEMAKVDDSFFH RLEESFLVEEDKKHERHPIFGNIVDEVAYHEKYPTIYHLRKKLVDSTDKA DLRLIYLALAHMIKFRGHFLIEGDLNPDNSDVDKLFIQLVQTYNQLFEEN PINASGVDAKAILSARLSKSRRLENLIAQLPGEKKNGLFGNLIALSLGLT PNFKSNFDLAEDAKLQLSKDTYDDDLDNLLAQIGDQYADLFLAAKNLSDA ILLSDILRVNTEITKAPLSASMIKRYDEHHQDLTLLKALVRQQLPEKYKE IFFDQSKNGYAGYIDGGASQEEFYKFIKPILEKMDGTEELLVKLNREDLL RKQRTFDNGSIPHQIHLGELHAILRRQEDFYPFLKDNREKIEKILTFRIP YYVGPLARGNSRFAWMTRKSEETITPWNFEEVVDKGASAQSFIERMTNFD KNLPNEKVLPKHSLLYEYFTVYNELTKVKYVTEGMRKPAFLSGEQKKAIV DLLFKTNRKVTVKQLKEDYFKKIECFDSVEISGVEDRFNASLGTYHDLLK IIKDKDFLDNEENEDILEDIVLTLTLFEDREMIEERLKTYAHLFDDKVMK QLKRRRYTGWGRLSRKLINGIRDKQSGKTILDFLKSDGFANRNFMQLIHD DSLTFKEDIQKAQVSGQGDSLHEHIANLAGSPAIKKGILQTVKVVDELVK VMGRHKPENIVIEMARENQTTQKGQKNSRERMKRIEEGIKELGSQILKEH PVENTQLQNEKLYLYYLQNGRDMYVDQELDINRLSDYDVDAIVPQSFLKD DSIDNKVLTRSDKNRGKSDNVPSEEVVKKMKNYWRQLLNAKLITQRKFDN LTKAERGGLSELDKAGFIKRQLVETRQITKHVAQILDSRMNTKYDENDKL IREVKVITLKSKLVSDFRKDFQFYKVREINNYHHAHDAYLNAVVGTALIK KYPKLESEFVYGDYKVYDVRKMIAKSEQEIGKATAKYFFYSNIMNFFKTE ITLANGEIRKRPLIETNGETGEIVWDKGRDFATVRKVLSMPQVNIVKKTE VQTGGFSKESILPKRNSDKLIARKKDWDPKKYGGFDSPTVAYSVLVVAKV EKGKSKKLKSVKELLGITIMERSSFEKNPIDFLEAKGYKEVKKDLIIKLP KYSLFELENGRKRMLASAGELQKGNELALPSKYVNFLYLASHYEKLKGSP EDNEQKQLFVEQHKHYLDEIIEQISEFSKRVILADANLDKVLSAYNKHRD KPIREQAENIIHLFTLTNLGAPAAFKYFDTTIDRKRYTSTKEVLDATLIH QSITGLYETRIDLSQLGGDGSPKKKRKVSSDYKDHDGDYKDHDIDYKDDD DK  FokI-dCas9 nucleotide sequence (FokI-G4S-dCas9- nls-3XFLAG) (SEQ ID NO: 25) ATGCAACTAGTCAAAAGTGAACTGGAGGAGAAGAAATCTGAACTTCGTCA TAAATTGAAATATGTGCCTCATGAATATATTGAATTAATTGAAATTGCCA GAAATTCCACTCAGGATAGAATTCTTGAAATGAAGGTAATGGAATTTTTT ATGAAAGTTTATGGATATAGAGGTAAACATTTGGGTGGATCAAGGAAACC GGACGGAGCAATTTATACTGTCGGATCTCCTATTGATTACGGTGTGATCG TGGATACTAAAGCTTATAGCGGAGGTTATAATCTGCCAATTGGCCAAGCA GATGAAATGCAACGATATGTCGAAGAAAATCAAACACGAAACAAACATAT CAACCCTAATGAATGGTGGAAAGTCTATCCATCTTCTGTAACGGAATTTA AGTTTTTATTTGTGAGTGGTCACTTTAAAGGAAACTACAAAGCTCAGCTT ACACGATTAAATCATATCACTAATTGTAATGGAGCTGTTCTTAGTGTAGA AGAGCTTTTAATTGGTGGAGAAATGATTAAAGCCGGCACATTAACCTTAG AGGAAGTCAGACGGAAATTTAATAACGGCGAGATAAACTTTGGTGGCGGT GGATCCGATAAAAAGTATTCTATTGGTTTAGCCATCGGCACTAATTCCGT TGGATGGGCTGTCATAACCGATGAATACAAAGTACCTTCAAAGAAATTTA AGGTGTTGGGGAACACAGACCGTCATTCGATTAAAAAGAATCTTATCGGT GCCCTCCTATTCGATAGTGGCGAAACGGCAGAGGCGACTCGCCTGAAACG AACCGCTCGGAGAAGGTATACACGTCGCAAGAACCGAATATGTTACTTAC AAGAAATTTTTAGCAATGAGATGGCCAAAGTTGACGATTCTTTCTTTCAC CGTTTGGAAGAGTCCTTCCTTGTCGAAGAGGACAAGAAACATGAACGGCA CCCCATCTTTGGAAACATAGTAGATGAGGTGGCATATCATGAAAAGTACC CAACGATTTATCACCTCAGAAAAAAGCTAGTTGACTCAACTGATAAAGCG GACCTGAGGTTAATCTACTTGGCTCTTGCCCATATGATAAAGTTCCGTGG GCACTTTCTCATTGAGGGTGATCTAAATCCGGACAACTCGGATGTCGACA AACTGTTCATCCAGTTAGTACAAACCTATAATCAGTTGTTTGAAGAGAAC CCTATAAATGCAAGTGGCGTGGATGCGAAGGCTATTCTTAGCGCCCGCCT CTCTAAATCCCGACGGCTAGAAAACCTGATCGCACAATTACCCGGAGAGA AGAAAAATGGGTTGTTCGGTAACCTTATAGCGCTCTCACTAGGCCTGACA CCAAATTTTAAGTCGAACTTCGACTTAGCTGAAGATGCCAAATTGCAGCT TAGTAAGGACACGTACGATGACGATCTCGACAATCTACTGGCACAAATTG GAGATCAGTATGCGGACTTATTTTTGGCTGCCAAAAACCTTAGCGATGCA ATCCTCCTATCTGACATACTGAGAGTTAATACTGAGATTACCAAGGCGCC GTTATCCGCTTCAATGATCAAAAGGTACGATGAACATCACCAAGACTTGA CACTTCTCAAGGCCCTAGTCCGTCAGCAACTGCCTGAGAAATATAAGGAA ATATTCTTTGATCAGTCGAAAAACGGGTACGCAGGTTATATTGACGGCGG AGCGAGTCAAGAGGAATTCTACAAGTTTATCAAACCCATATTAGAGAAGA TGGATGGGACGGAAGAGTTGCTTGTAAAACTCAATCGCGAAGATCTACTG CGAAAGCAGCGGACTTTCGACAACGGTAGCATTCCACATCAAATCCACTT AGGCGAATTGCATGCTATACTTAGAAGGCAGGAGGATTTTTATCCGTTCC TCAAAGACAATCGTGAAAAGATTGAGAAAATCCTAACCTTTCGCATACCT TACTATGTGGGACCCCTGGCCCGAGGGAACTCTCGGTTCGCATGGATGAC AAGAAAGTCCGAAGAAACGATTACTCCATGGAATTTTGAGGAAGTTGTCG ATAAAGGTGCGTCAGCTCAATCGTTCATCGAGAGGATGACCAACTTTGAC AAGAATTTACCGAACGAAAAAGTATTGCCTAAGCACAGTTTACTTTACGA GTATTTCACAGTGTACAATGAACTCACGAAAGTTAAGTATGTCACTGAGG GCATGCGTAAACCCGCCTTTCTAAGCGGAGAACAGAAGAAAGCAATAGTA GATCTGTTATTCAAGACCAACCGCAAAGTGACAGTTAAGCAATTGAAAGA GGACTACTTTAAGAAAATTGAATGCTTCGATTCTGTCGAGATCTCCGGGG TAGAAGATCGATTTAATGCGTCACTTGGTACGTATCATGACCTCCTAAAG ATAATTAAAGATAAGGACTTCCTGGATAACGAAGAGAATGAAGATATCTT AGAAGATATAGTGTTGACTCTTACCCTCTTTGAAGATCGGGAAATGATTG AGGAAAGACTAAAAACATACGCTCACCTGTTCGACGATAAGGTTATGAAA CAGTTAAAGAGGCGTCGCTATACGGGCTGGGGACGATTGTCGCGGAAACT TATCAACGGGATAAGAGACAAGCAAAGTGGTAAAACTATTCTCGATTTTC TAAAGAGCGACGGCTTCGCCAATAGGAACTTTATGCAGCTGATCCATGAT GACTCTTTAACCTTCAAAGAGGATATACAAAAGGCACAGGTTTCCGGACA AGGGGACTCATTGCACGAACATATTGCGAATCTTGCTGGTTCGCCAGCCA TCAAAAAGGGCATACTCCAGACAGTCAAAGTAGTGGATGAGCTAGTTAAG GTCATGGGACGTCACAAACCGGAAAACATTGTAATCGAGATGGCACGCGA AAATCAAACGACTCAGAAGGGGCAAAAAAACAGTCGAGAGCGGATGAAGA GAATAGAAGAGGGTATTAAAGAACTGGGCAGCCAGATCTTAAAGGAGCAT CCTGTGGAAAATACCCAATTGCAGAACGAGAAACTTTACCTCTATTACCT ACAAAATGGAAGGGACATGTATGTTGATCAGGAACTGGACATAAACCGTT TATCTGATTACGACGTCGATGCCATTGTACCCCAATCCTTTTTGAAGGAC GATTCAATCGACAATAAAGTGCTTACACGCTCGGATAAGAACCGAGGGAA AAGTGACAATGTTCCAAGCGAGGAAGTCGTAAAGAAAATGAAGAACTATT GGCGGCAGCTCCTAAATGCGAAACTGATAACGCAAAGAAAGTTCGATAAC TTAACTAAAGCTGAGAGGGGTGGCTTGTCTGAACTTGACAAGGCCGGATT TATTAAACGTCAGCTCGTGGAAACCCGCCAAATCACAAAGCATGTTGCAC AGATACTAGATTCCCGAATGAATACGAAATACGACGAGAACGATAAGCTG ATTCGGGAAGTCAAAGTAATCACTTTAAAGTCAAAATTGGTGTCGGACTT CAGAAAGGATTTTCAATTCTATAAAGTTAGGGAGATAAATAACTACCACC ATGCGCACGACGCTTATCTTAATGCCGTCGTAGGGACCGCACTCATTAAG AAATACCCGAAGCTAGAAAGTGAGTTTGTGTATGGTGATTACAAAGTTTA TGACGTCCGTAAGATGATCGCGAAAAGCGAACAGGAGATAGGCAAGGCTA CAGCCAAATACTTCTTTTATTCTAACATTATGAATTTCTTTAAGACGGAA ATCACTCTGGCAAACGGAGAGATACGCAAACGACCTTTAATTGAAACCAA TGGGGAGACAGGTGAAATCGTATGGGATAAGGGCCGGGACTTCGCGACGG TGAGAAAAGTTTTGTCCATGCCCCAAGTCAACATAGTAAAGAAAACTGAG GTGCAGACCGGAGGGTTTTCAAAGGAATCGATTCTTCCAAAAAGGAATAG TGATAAGCTCATCGCTCGTAAAAAGGACTGGGACCCGAAAAAGTACGGTG GCTTCGATAGCCCTACAGTTGCCTATTCTGTCCTAGTAGTGGCAAAAGTT GAGAAGGGAAAATCCAAGAAACTGAAGTCAGTCAAAGAATTATTGGGGAT AACGATTATGGAGCGCTCGTCTTTTGAAAAGAACCCCATCGACTTCCTTG AGGCGAAAGGTTACAAGGAAGTAAAAAAGGATCTCATAATTAAACTACCA AAGTATAGTCTGTTTGAGTTAGAAAATGGCCGAAAACGGATGTTGGCTAG CGCCGGAGAGCTTCAAAAGGGGAACGAACTCGCACTACCGTCTAAATACG TGAATTTCCTGTATTTAGCGTCCCATTACGAGAAGTTGAAAGGTTCACCT GAAGATAACGAACAGAAGCAACTTTTTGTTGAGCAGCACAAACATTATCT CGACGAAATCATAGAGCAAATTTCGGAATTCAGTAAGAGAGTCATCCTAG CTGATGCCAATCTGGACAAAGTATTAAGCGCATACAACAAGCACAGGGAT AAACCCATACGTGAGCAGGCGGAAAATATTATCCATTTGTTTACTCTTAC CAACCTCGGCGCTCCAGCCGCATTCAAGTATTTTGACACAACGATAGATC GCAAACGATACACTTCTACCAAGGAGGTGCTAGACGCGACACTGATTCAC CAATCCATCACGGGATTATATGAAACTCGGATAGATTTGTCACAGCTTGG GGGTGACGGATCCCCCAAGAAGAAGAGGAAAGTCTCGAGCGACTACAAAG ACCATGACGGTGATTATAAAGATCATGACATCGATTACAAGGATGACGAT GACAAGTGA 

Alternatively, a human codon optimized version of the construct was used, which contained both N- and C-terminal nuclear localization signals, as shown below.

Nls-FokI-dCas9-nls amino acid sequence (SEQ ID NO: 26) MPKKKRKVSSQLVKSELEEKKSELRHKLKYVPHEYIELIEIARNSTQDR ILEMKVMEFFMKVYGYRGKHLGGSRKPDGAIYTVGSPIDYGVIVDTKAY SGGYNLPIGQADEMQRYVEENQTRNKHINPNEWWKVYPSSVTEFKFLFV SGHFKGNYKAQLTRLNHITNCNGAVLSVEELLIGGEMIKAGTLTLEEVR RKFNNGEINFGGGGSDKKYSIGLAIGTNSVGWAVITDEYKVPSKKFKVL GNTDRHSIKKNLIGALLFDSGETAEATRLKRTARRRYTRRKNRICYLQE IFSNEMAKVDDSFFHRLEESFLVEEDKKHERHPIFGNIVDEVAYHEKYP TIYHLRKKLVDSTDKADLRLIYLALAHMIKFRGHFLIEGDLNPDNSDVD KLFIQLVQTYNQLFEENPINASGVDAKAILSARLSKSRRLENLIAQLPG EKKNGLFGNLIALSLGLTPNFKSNFDLAEDAKLQLSKDTYDDDLDNLLA QIGDQYADLFLAAKNLSDAILLSDILRVNTEITKAPLSASMIKRYDEHH QDLTLLKALVRQQLPEKYKEIFFDQSKNGYAGYIDGGASQEEFYKFIKP ILEKMDGTEELLVKLNREDLLRKQRTFDNGSIPHQIHLGELHAILRRQE DFYPFLKDNREKIEKILTFRIPYYVGPLARGNSRFAWMTRKSEETITPW NFEEVVDKGASAQSFIERMTNFDKNLPNEKVLPKHSLLYEYFTVYNELT KVKYVTEGMRKPAFLSGEQKKAIVDLLFKTNRKVTVKQLKEDYFKKIEC FDSVEISGVEDRFNASLGTYHDLLKIIKDKDFLDNEENEDILEDIVLTL TLFEDREMIEERLKTYAHLFDDKVMKQLKRRRYTGWGRLSRKLINGIRD KQSGKTILDFLKSDGFANRNFMQLIHDDSLTFKEDIQKAQVSGQGDSLH EHIANLAGSPAIKKGILQTVKVVDELVKVMGRHKPENIVIEMARENQTT QKGQKNSRERMKRIEEGIKELGSQILKEHPVENTQLQNEKLYLYYLQNG RDMYVDQELDINRLSDYDVDAIVPQSFLKDDSIDNKVLTRSDKNRGKSD NVPSEEVVKKMKNYWRQLLNAKLITQRKFDNLTKAERGGLSELDKAGFI KRQLVETRQITKHVAQILDSRMNTKYDENDKLIREVKVITLKSKLVSDF RKDFQFYKVREINNYHHAHDAYLNAVVGTALIKKYPKLESEFVYGDYKV YDVRKMIAKSEQEIGKATAKYFFYSNIMNFFKTEITLANGEIRKRPLIE TNGETGEIVWDKGRDFATVRKVLSMPQVNIVKKTEVQTGGFSKESILPK RNSDKLIARKKDWDPKKYGGFDSPTVAYSVLVVAKVEKGKSKKLKSVKE LLGITIMERSSFEKNPIDFLEAKGYKEVKKDLIIKLPKYSLFELENGRK RMLASAGELQKGNELALPSKYVNFLYLASHYEKLKGSPEDNEQKQLFVE QHKHYLDEIIEQISEFSKRVILADANLDKVLSAYNKHRDKPIREQAENI IHLFTLTNLGAPAAFKYFDTTIDRKRYTSTKEVLDATLIHQSITGLYET RIDLSQLGGDGSPKKKRKVSSDYKDHDGDYKDHDIDYKDDDDK  Nls-FokI-dCas9-nls nucleotide sequence (SEQ ID NO: 27) ATGCCTAAGAAGAAGCGGAAGGTGAGCAGCCAACTTGTGAAGTCTGAAC TCGAGGAGAAAAAATCAGAGTTGAGACACAAGTTGAAGTACGTGCCACA CGAATACATCGAGCTTATCGAGATCGCCAGAAACAGTACCCAGGATAGG ATCCTTGAGATGAAAGTCATGGAGTTCTTTATGAAGGTCTACGGTTATA GAGGAAAGCACCTTGGCGGTAGCAGAAAGCCCGATGGCGCCATCTATAC TGTCGGATCTCCTATCGATTATGGGGTGATCGTGGATACCAAAGCTTAC TCAGGCGGGTACAACTTGCCCATAGGACAAGCCGACGAGATGCAGCGGT ATGTCGAAGAGAACCAGACGCGCAACAAGCACATCAACCCCAATGAATG GTGGAAAGTGTACCCAAGTAGTGTGACTGAGTTCAAGTTCCTGTTTGTC TCCGGCCACTTTAAGGGCAATTATAAAGCTCAGCTCACTAGACTCAATC ACATCACAAACTGCAACGGAGCTGTGTTGTCAGTGGAGGAGCTCCTGAT TGGAGGCGAGATGATCAAAGCCGGCACCCTTACACTGGAGGAGGTGCGG CGGAAGTTCAACAATGGAGAGATCAACTTCGGTGGCGGTGGATCCGATA AAAAGTATTCTATTGGTTTAGCCATCGGCACTAATTCCGTTGGATGGGC TGTCATAACCGATGAATACAAAGTACCTTCAAAGAAATTTAAGGTGTTG GGGAACACAGACCGTCATTCGATTAAAAAGAATCTTATCGGTGCCCTCC TATTCGATAGTGGCGAAACGGCAGAGGCGACTCGCCTGAAACGAACCGC TCGGAGAAGGTATACACGTCGCAAGAACCGAATATGTTACTTACAAGAA ATTTTTAGCAATGAGATGGCCAAAGTTGACGATTCTTTCTTTCACCGTT TGGAAGAGTCCTTCCTTGTCGAAGAGGACAAGAAACATGAACGGCACCC CATCTTTGGAAACATAGTAGATGAGGTGGCATATCATGAAAAGTACCCA ACGATTTATCACCTCAGAAAAAAGCTAGTTGACTCAACTGATAAAGCGG ACCTGAGGTTAATCTACTTGGCTCTTGCCCATATGATAAAGTTCCGTGG GCACTTTCTCATTGAGGGTGATCTAAATCCGGACAACTCGGATGTCGAC AAACTGTTCATCCAGTTAGTACAAACCTATAATCAGTTGTTTGAAGAGA ACCCTATAAATGCAAGTGGCGTGGATGCGAAGGCTATTCTTAGCGCCCG CCTCTCTAAATCCCGACGGCTAGAAAACCTGATCGCACAATTACCCGGA GAGAAGAAAAATGGGTTGTTCGGTAACCTTATAGCGCTCTCACTAGGCC TGACACCAAATTTTAAGTCGAACTTCGACTTAGCTGAAGATGCCAAATT GCAGCTTAGTAAGGACACGTACGATGACGATCTCGACAATCTACTGGCA CAAATTGGAGATCAGTATGCGGACTTATTTTTGGCTGCCAAAAACCTTA GCGATGCAATCCTCCTATCTGACATACTGAGAGTTAATACTGAGATTAC CAAGGCGCCGTTATCCGCTTCAATGATCAAAAGGTACGATGAACATCAC CAAGACTTGACACTTCTCAAGGCCCTAGTCCGTCAGCAACTGCCTGAGA AATATAAGGAAATATTCTTTGATCAGTCGAAAAACGGGTACGCAGGTTA TATTGACGGCGGAGCGAGTCAAGAGGAATTCTACAAGTTTATCAAACCC ATATTAGAGAAGATGGATGGGACGGAAGAGTTGCTTGTAAAACTCAATC GCGAAGATCTACTGCGAAAGCAGCGGACTTTCGACAACGGTAGCATTCC ACATCAAATCCACTTAGGCGAATTGCATGCTATACTTAGAAGGCAGGAG GATTTTTATCCGTTCCTCAAAGACAATCGTGAAAAGATTGAGAAAATCC TAACCTTTCGCATACCTTACTATGTGGGACCCCTGGCCCGAGGGAACTC TCGGTTCGCATGGATGACAAGAAAGTCCGAAGAAACGATTACTCCATGG AATTTTGAGGAAGTTGTCGATAAAGGTGCGTCAGCTCAATCGTTCATCG AGAGGATGACCAACTTTGACAAGAATTTACCGAACGAAAAAGTATTGCC TAAGCACAGTTTACTTTACGAGTATTTCACAGTGTACAATGAACTCACG AAAGTTAAGTATGTCACTGAGGGCATGCGTAAACCCGCCTTTCTAAGCG GAGAACAGAAGAAAGCAATAGTAGATCTGTTATTCAAGACCAACCGCAA AGTGACAGTTAAGCAATTGAAAGAGGACTACTTTAAGAAAATTGAATGC TTCGATTCTGTCGAGATCTCCGGGGTAGAAGATCGATTTAATGCGTCAC TTGGTACGTATCATGACCTCCTAAAGATAATTAAAGATAAGGACTTCCT GGATAACGAAGAGAATGAAGATATCTTAGAAGATATAGTGTTGACTCTT ACCCTCTTTGAAGATCGGGAAATGATTGAGGAAAGACTAAAAACATACG CTCACCTGTTCGACGATAAGGTTATGAAACAGTTAAAGAGGCGTCGCTA TACGGGCTGGGGACGATTGTCGCGGAAACTTATCAACGGGATAAGAGAC AAGCAAAGTGGTAAAACTATTCTCGATTTTCTAAAGAGCGACGGCTTCG CCAATAGGAACTTTATGCAGCTGATCCATGATGACTCTTTAACCTTCAA AGAGGATATACAAAAGGCACAGGTTTCCGGACAAGGGGACTCATTGCAC GAACATATTGCGAATCTTGCTGGTTCGCCAGCCATCAAAAAGGGCATAC TCCAGACAGTCAAAGTAGTGGATGAGCTAGTTAAGGTCATGGGACGTCA CAAACCGGAAAACATTGTAATCGAGATGGCACGCGAAAATCAAACGACT CAGAAGGGGCAAAAAAACAGTCGAGAGCGGATGAAGAGAATAGAAGAGG GTATTAAAGAACTGGGCAGCCAGATCTTAAAGGAGCATCCTGTGGAAAA TACCCAATTGCAGAACGAGAAACTTTACCTCTATTACCTACAAAATGGA AGGGACATGTATGTTGATCAGGAACTGGACATAAACCGTTTATCTGATT ACGACGTCGATGCCATTGTACCCCAATCCTTTTTGAAGGACGATTCAAT CGACAATAAAGTGCTTACACGCTCGGATAAGAACCGAGGGAAAAGTGAC AATGTTCCAAGCGAGGAAGTCGTAAAGAAAATGAAGAACTATTGGCGGC AGCTCCTAAATGCGAAACTGATAACGCAAAGAAAGTTCGATAACTTAAC TAAAGCTGAGAGGGGTGGCTTGTCTGAACTTGACAAGGCCGGATTTATT AAACGTCAGCTCGTGGAAACCCGCCAAATCACAAAGCATGTTGCACAGA TACTAGATTCCCGAATGAATACGAAATACGACGAGAACGATAAGCTGAT TCGGGAAGTCAAAGTAATCACTTTAAAGTCAAAATTGGTGTCGGACTTC AGAAAGGATTTTCAATTCTATAAAGTTAGGGAGATAAATAACTACCACC ATGCGCACGACGCTTATCTTAATGCCGTCGTAGGGACCGCACTCATTAA GAAATACCCGAAGCTAGAAAGTGAGTTTGTGTATGGTGATTACAAAGTT TATGACGTCCGTAAGATGATCGCGAAAAGCGAACAGGAGATAGGCAAGG CTACAGCCAAATACTTCTTTTATTCTAACATTATGAATTTCTTTAAGAC GGAAATCACTCTGGCAAACGGAGAGATACGCAAACGACCTTTAATTGAA ACCAATGGGGAGACAGGTGAAATCGTATGGGATAAGGGCCGGGACTTCG CGACGGTGAGAAAAGTTTTGTCCATGCCCCAAGTCAACATAGTAAAGAA AACTGAGGTGCAGACCGGAGGGTTTTCAAAGGAATCGATTCTTCCAAAA AGGAATAGTGATAAGCTCATCGCTCGTAAAAAGGACTGGGACCCGAAAA AGTACGGTGGCTTCGATAGCCCTACAGTTGCCTATTCTGTCCTAGTAGT GGCAAAAGTTGAGAAGGGAAAATCCAAGAAACTGAAGTCAGTCAAAGAA TTATTGGGGATAACGATTATGGAGCGCTCGTCTTTTGAAAAGAACCCCA TCGACTTCCTTGAGGCGAAAGGTTACAAGGAAGTAAAAAAGGATCTCAT AATTAAACTACCAAAGTATAGTCTGTTTGAGTTAGAAAATGGCCGAAAA CGGATGTTGGCTAGCGCCGGAGAGCTTCAAAAGGGGAACGAACTCGCAC TACCGTCTAAATACGTGAATTTCCTGTATTTAGCGTCCCATTACGAGAA GTTGAAAGGTTCACCTGAAGATAACGAACAGAAGCAACTTTTTGTTGAG CAGCACAAACATTATCTCGACGAAATCATAGAGCAAATTTCGGAATTCA GTAAGAGAGTCATCCTAGCTGATGCCAATCTGGACAAAGTATTAAGCGC ATACAACAAGCACAGGGATAAACCCATACGTGAGCAGGCGGAAAATATT ATCCATTTGTTTACTCTTACCAACCTCGGCGCTCCAGCCGCATTCAAGT ATTTTGACACAACGATAGATCGCAAACGATACACTTCTACCAAGGAGGT GCTAGACGCGACACTGATTCACCAATCCATCACGGGATTATATGAAACT CGGATAGATTTGTCACAGCTTGGGGGTGACGGATCCCCCAAGAAGAAGA GGAAAGTCTCGAGCGACTACAAAGACCATGACGGTGATTATAAAGATCA TGACATCGATTACAAGGATGACGATGACAAGTGA 

Tissue Culture and Transfections

All cell culture experiments were carried out in HEK 293 cells, U2OS cells, or in U2OS cells harboring a stably integrated, single-copy, destabilized EGFP gene (U2OS.EGFP cells). Cell lines were cultured in Advanced DMEM (Life Technologies) supplemented with 10% FBS, 2 mM GlutaMax (Life Technologies) and penicillin/streptomycin at 37 C with 5% CO2. Additionally, U2OS.EGFP cells were cultured in the presence of 400 μg/ml of G418.

U2OS cells and U2OS.EGFP cells were transfected using the DN-100 program of a Lonza 4D-Nucleofector according to the manufacturer's instructions. In initial FokI-dCas9 activity screens and focused spacer length analysis experiments, 750 ng of pCAG-Csy4-FokI-dCas9-nls nuclease plasmid and 250 ng of gRNA encoding plasmids were transfected together with 50 ng tdTomato expression plasmid (Clontech) as a transfection control. In all other experiments in U2OS and U2OS.EGFP cells, 975 ng of human codon optimized pCAG-Csy4-T2A-nls-hFokI-dCas9-nls (SQT1601) or pCAG-Cas9-D10A nickase (NW3) were transfected along with 325 ng of gRNA vector and 10 ng of Td tomato expression plasmid and analyzed 3 days after transfection. HEK293 cells were transfected with 750 ng of nuclease plasmid, 250 ng of gRNA expression plasmid and 10 ng of Td tomato, using Lipofectamine (Life Technologies) according to the manufacturer's instructions and analyzed for NHEJ-mediated mutagenesis 3 days after transfection.

Single transfections were performed for the initial spacer activity screen, and duplicate transfections for the focused spacer length analysis. All other transfections were performed in triplicate.

EGFP Disruption Assay

The EGFP disruption assay was performed as previously described (see Example 1 and Reyon et al., Nat Biotech 30, 460-465 (2012)) using U2OS.EGFP reporter cells. Cells were assayed for EGFP and tdTomato expression using an BD Biosciences LSR II or Fortessa FACS analyzer.

Quantification of Nuclease- or Nickase-Induced Mutation Rates by T7EI Assay

T7E1 assays were performed as previously described (Reyon et al., Nat Biotech 30, 460-465 (2012)). Briefly, genomic DNA was isolated 72 hours post transfection using the Agencourt DNAdvance Genomic DNA Isolation kit (Beckman Coulter Genomics) according to the manufacturer's instructions with a Sciclone G3 liquid-handling workstation (Caliper). PCR reactions to amplify genomic loci were performed using Phusion Hot-start Flex DNA polymerase (New England Biolabs). Samples were amplified using a two-step protocol (98° C., 30 sec; (98° C., 7 sec; 72° C., 30 sec)×35; 72° C., 5 min) or a touchdown PCR protocol ((98° C., 10 s; 72-62° C., −1° C./cycle, 15 s; 72° C., 30 s)×10 cycles, (98° C., 10 s; 62° C., 15 s; 72° C., 30 s)×25 cycles). 200 ng of purified PCR amplicons were denatured, hybridized, and treated with T7 Endonuclease I (New England Biolabs). Mutation frequency was quantified using a Qiaxcel capillary electrophoresis instrument (Qiagen) as previously described (Reyon et al., Nat Biotech 30, 460-465 (2012)).

Sanger Sequencing of Mutagenized Genomic DNA

The same purified PCR products used for T7EI assay were Topo-cloned (Life Technologies) and plasmid DNA of individual clones was isolated and sequenced using an M13 reverse primer (5′-GTAAAACGACGGCCAG-3′; SEQ ID NO:19).

Illumina Library Preparation and Analysis

Short 200-350 bp PCR products were amplified using Phusion Hot-start FLEX DNA polymerase. PCR products were purified using Ampure XP beads (Beckman Coulter Genomics) according to manufacturer's instructions. Dual-indexed TruSeq Illumina deep sequencing libraries were prepared using a high-throughput library preparation system (Kapa Biosystems) on a Sciclone G3 liquid-handling workstation. Final adapter-ligated libraries were quantified using a Qiaxcel capillary electrophoresis instrument (Qiagen). 150 bp paired end sequencing was performed on an Illumina MiSeq Sequencer by the Dana-Farber Cancer Institute Molecular Biology Core.

MiSeq paired-end reads were mapped to human genome reference GChr37 using bwa. Reads with an average quality score >30 were analyzed for insertion or deletion mutations that overlapped the intended target or candidate off-target nuclease binding site. Mutation analyses were conducted using the Genome Analysis Toolkit (GATK) and Python.

Off-Target Search Algorithm:

A target-site matching algorithm was implemented that looks for matches with less than a specified number of mismatches in a sliding window across the human genome.

Example 2a. Rationale for Designing Dimeric RNA-Guided Nucleases

It was hypothesized that a single platform combining the specificity advantages of dimerization with the ease of Cas9 targeting could be developed. To do this, the well-characterized, dimerization-dependent FokI nuclease domain was fused to a RNA-guided catalytically inactive Cas9 (dCas9) protein. It was hoped that, like FokI-containing ZFNs and TALENs, dimers of these fusions might mediate sequence-specific DNA cleavage when bound to target sites composed of two “half-sites” with a certain length “spacer” sequence between them (FIG. 4A). Such fusions were hypothesized to have enhanced specificity because they should require two gRNAs for activity (FIG. 4A) and because a single gRNA would presumably be too inefficient or unable to recruit the two FokI-containing fusion proteins required for DNA cleavage. It was hypothesized that such a dimeric system would show improved specificity relative to standard monomeric Cas9 nucleases and also would potentially possess important specificity advantages over the paired nickase system in which single nickases can still exert unwanted mutagenic effects.

Example 2b. Multiplex Expression of gRNAs without 5′-End Nucleotide Limitations

The targeting range for a dimeric RNA-guided nuclease would be low using existing gRNA expression methods. Two sequence requirements typically restrict the targeting range of a dCas9 monomer: the requirement for a PAM sequence of 5′-NGG that is specified by the dCas9 and a requirement for a G nucleotide at the 5′ end of the gRNA imposed by the use of a U6 promoter in most expression vectors. If, however, the requirement for the 5′ G in the gRNA could be relieved, then the targeting range would improve by 16-fold.

To develop a multiplex system that would allow for the expression of gRNAs with any 5′ nucleotide, a plasmid was constructed from which two gRNAs, each flanked by cleavage sites for the Csy4 ribonuclease (Haurwitz et al., Science 329, 1355-1358 (2010)), can be expressed within a single RNA transcribed from a U6 promoter (FIG. 4B). Csy4 would be expected to process this transcript thereby releasing the two gRNAs. Based on the known mechanism of Csy4-mediated cleavage ((Haurwitz et al., Science 329, 1355-1358 (2010); Sternberg et al., RNA 18, 661-672 (2012)), each processed gRNA should retain a Csy4 recognition site on its 3′ end with a Csy4 protein bound to that site (FIG. 4B). In this configuration, it should be possible to express gRNAs with any 5′ nucleotide. This system was tested by using it to express two gRNAs targeted to sites within the EGFP reporter gene. Co-expression of this transcript together with Csy4 and Cas9 nucleases in human cells led to the introduction of indel mutations at both EGFP target sites as well as of deletion of the sequence between these sites (FIG. 4C). These experiments suggest that both gRNAs are being processed from the single parental RNA transcript and both are capable of directing Cas9 nuclease activities in human cells.

Example 2c. Construction and Optimization of Dimeric RNA-Guided Nucleases

Two different hybrid proteins harboring the FokI nuclease domain and the dCas9 protein were constructed: one in which the FokI nuclease domain is fused to the carboxy-terminus of dCas9 (dCas9-FokI) and the other in which it is fused to the amino-terminus (FokI-dCas9) (FIG. 5A). The dCas9-FokI protein is analogous in architecture to ZFNs and TALENs (FIG. 5A). To ascertain whether either or both of these fusions could mediate site-specific cleavage of DNA, a well-established human cell-based assay that can rapidly and easily quantify the introduction of NHEJ-mediated indels into an EGFP reporter gene was used (the EGFP disruption assay described above in Example 1). Because the geometry of the half-sites required for efficient cleavage was not known, 60 pairs of gRNAs targeted to various sites in EGFP were designed. The two half-sites targeted by each of these gRNA pairs were oriented such that both of their PAM sequences are either directly adjacent to the spacer sequence (the “PAM in” orientation) or positioned at the outer boundaries of the full-length target site (the “PAM out” orientation) (FIG. 5B). In addition, the spacer sequence was also varied in length from 0 to 31 bps (FIG. 5B and Table 2).

TABLE 2 FokI- Edge- dCas9 Target to- EGFP Start SEQ SEQ edge Pair Position Sequence ID Sequence ID ′spacer′ # Name (+) (+) sites NO: (-) sites NO: distance PAM  1 EGFP site  74 GAGCTGGACGGCGACGTAAACG  28. CGCCGGACACGCTGAACTTGTGG  29.  0 in 1 G  2 EGFP site 174 CCGGCAAGCTGCCCGTGCCCTG  30. GGTCAGGGTGGTCACGAGGGTGG  31.  1 in 2 G  3 EGFP site  37 CGAGGAGCTGTTCACCGGGGTG  32. CCGTCCAGCTCGACCAGGATGGG  33.  2 in 3 G  4 EGFP site  37 CGAGGAGCTGTTCACCGGGGTG  34. GCCGTCCAGCTCGACCAGGATGG  35.  3 in 4 G  5 EGFP site 174 CCGGCAAGCTGCCCGTGCCCTG  36. GTAGGTCAGGGTGGTCACGAGGG  37.  4 in 5 G  6 EGFP site  34 GGGCGAGGAGCTGTTCACCGGG  38. CCGTCCAGCTCGACCAGGATGGG  39.  5 in 6 G  7 EGFP site  33 AGGGCGAGGAGCTGTTCACCGG  40. CCGTCCAGCTCGACCAGGATGGG  41.  6 in 7 G  8 EGFP site  32 AAGGGCGAGGAGCTGTTCACCG  42. CCGTCCAGCTCGACCAGGATGGG  43.  7 in 8 G  9 EGFP site  32 AAGGGCGAGGAGCTGTTCACCG  44. GCCGTCCAGCTCGACCAGGATGG  45.  8 in 9 G 10 EGFP site 106 CAGCGTGTCCGGCGAGGGCGAG  46. CTTCAGGGTCAGCTTGCCGTAGG  47.  9 in 10 G 11 EGFP site  34 GGGCGAGGAGCTGTTCACCGGG  48. CGTCGCCGTCCAGCTCGACCAGG  49. 10 in 11 G 12 EGFP site  33 AGGGCGAGGAGCTGTTCACCGG  50. CGTCGCCGTCCAGCTCGACCAGG  51. 11 in 12 G 13 EGFP site  32 AAGGGCGAGGAGCTGTTCACCG  52. CGTCGCCGTCCAGCTCGACCAGG  53. 12 in 13 G 14 EGFP site 155 CTGAAGTTCATCTGCACCACCG  54. GTGGTCACGAGGGTGGGCCAGGG  55. 13 in 14 G 15 EGFP site 101 AAGTTCAGCGTGTCCGGCGAGG  56. CTTCAGGGTCAGCTTGCCGTAGG  57. 14 in 15 G 16 EGFP site 100 CAAGTTCAGCGTGTCCGGCGAG  58. CTTCAGGGTCAGCTTGCCGTAGG  59. 15 in 16 G 17 EGFP site  58 GGTGCCCATCCTGGTCGAGCTG  60. CGCCGGACACGCTGAACTTGTGG  61. 16 in 17 G 18 EGFP site  74 GAGCTGGACGGCGACGTAAACG  62. GGCATCGCCCTCGCCCTCGCCGG  63. 17 in 18 G 19 EGFP site 307 GGAGCGCACCATCTTCTTCAAG  64. CTCGAACTTCACCTCGGCGCGGG  65. 18 in 19 G 20 EGFP site 155 CTGAAGTTCATCTGCACCACCG  66. GTCAGGGTGGTCACGAGGGTGGG  67. 19 in 20 G 21 EGFP site  95 GGCCACAAGTTCAGCGTGTCCG  68. CTTCAGGGTCAGCTTGCCGTAGG  69. 20 in 21 G 22 EGFP site 203 CTCGTGACCACCCTGACCTACG  70. CGTGCTGCTTCATGTGGTCGGGG  71. 21 in 22 G 23 EGFP site 174 CCGGCAAGCTGCCCGTGCCCTG  72. GCTGAAGCACTGCACGCCGTAGG  73. 22 in 23 G 24 EGFP site 107 AGCGTGTCCGGCGAGGGCGAGG  74. GGTGGTGCAGATGAACTTCAGGG  75. 23 in 24 G 25 EGFP site 106 CAGCGTGTCCGGCGAGGGCGAG  76. GGTGGTGCAGATGAACTTCAGGG  77. 24 in 25 G 26 EGFP site  49 CACCGGGGTGGTGCCCATCCTG  78. CGCCGGACACGCTGAACTTGTGG  79. 25 in 26 G 27 EGFP site 122 GGCGAGGGCGATGCCACCTACG  80. GGGCACGGGCAGCTTGCCGGTGG  81. 26 in 27 G 28 EGFP site 203 CTCGTGACCACCCTGACCTACG  82. AGAAGTCGTGCTGCTTCATGTGG  83. 27 in 28 G 29 EGFP site 337 CAACTACAAGACCCGCGCCGAG  84. CGATGCCCTTCAGCTCGATGCGG  85. 28 in 29 G 30 EGFP site  62 CCCATCCTGGTCGAGCTGGACG  86. GGCATCGCCCTCGCCCTCGCCGG  87. 29 in 30 G 31 EGFP site 100 CAAGTTCAGCGTGTCCGGCGAG  88. GGTGGTGCAGATGAACTTCAGGG  89. 30 in 31 G 32 EGFP site  74 GAGCTGGACGGCGACGTAAACG  90. GACCAGGATGGGCACCACCCCGG  91.  0 out 32 G 33 EGFP site 314 ACCATCTTCTTCAAGGACGACG  92. CGCTCCTGGACGTAGCCTTCGGG  93.  1 out 33 G 34 EGFP site 122 GGCGAGGGCGATGCCACCTACG  94. CGCCGGACACGCTGAACTTGTGG  95.  2 out 34 G 35 EGFP site 275 TTCAAGTCCGCCATGCCCGAAG  96. GTCGTGCTGCTTCATGTGGTCGG  97.  3 out 35 G 36 EGFP site 275 TTCAAGTCCGCCATGCCCGAAG  98. TCGTGCTGCTTCATGTGGTCGGG  99.  4 out 36 G 37 EGFP site  95 GGCCACAAGTTCAGCGTGTCCG 100. CGTCGCCGTCCAGCTCGACCAGG 101.  5 out 37 G 38 EGFP site 203 CTCGTGACCACCCTGACCTACG 102. CCAGGGCACGGGCAGCTTGCCGG 103.  6 out 38 G 39 EGFP site 463 CAGCCACAACGTCTATATCATG 104. TGTACTCCAGCTTGTGCCCCAGG 105.  7 out 39 G 40 EGFP site  95 GGCCACAAGTTCAGCGTGTCCG 106. GCCGTCCAGCTCGACCAGGATGG 107.  9 out 40 G 41 EGFP site  95 GGCCACAAGTTCAGCGTGTCCG 108. CCGTCCAGCTCGACCAGGATGGG 109. 10 out 41 G 42 EGFP site 101 AAGTTCAGCGTGTCCGGCGAGG 110. CGTCGCCGTCCAGCTCGACCAGG 111. 11 out 42 G 43 EGFP site 350 CGCGCCGAGGTGAAGTTCGAGG 112. GCCGTCGTCCTTGAAGAAGATGG 113. 12 out 43 G 44 EGFP site 174 CCGGCAAGCTGCCCGTGCCCTG 114. CTTCAGGGTCAGCTTGCCGTAGG 115. 13 out 44 G 45 EGFP site 100 CAAGTTCAGCGTGTCCGGCGAG 116. GCCGTCCAGCTCGACCAGGATGG 117. 14 out 45 G 46 EGFP site 100 CAAGTTCAGCGTGTCCGGCGAG 118. CCGTCCAGCTCGACCAGGATGGG 119. 15 out 46 G 47 EGFP site 101 AAGTTCAGCGTGTCCGGCGAGG 120. CCGTCCAGCTCGACCAGGATGGG 121. 16 out 47 G 48 EGFP site 107 AGCGTGTCCGGCGAGGGCGAGG 122. CGTCGCCGTCCAGCTCGACCAGG 123. 17 out 48 G 49 EGFP site 155 CTGAAGTTCATCTGCACCACCG 124. GGCATCGCCCTCGCCCTCGCCGG 125. 18 out 49 G 50 EGFP site 106 CAGCGTGTCCGGCGAGGGCGAG 126. GCCGTCCAGCTCGACCAGGATGG 127. 20 out 50 G 51 EGFP site  95 GGCCACAAGTTCAGCGTGTCCG 128. GACCAGGATGGGCACCACCCCGG 129. 21 out 51 G 52 EGFP site 107 AGCGTGTCCGGCGAGGGCGAGG 130. CCGTCCAGCTCGACCAGGATGGG 131. 22 out 52 G 53 EGFP site 337 CAACTACAAGACCCGCGCCGAG 132. GCGCTCCTGGACGTAGCCTTCGG 133. 23 out 53 G 54 EGFP site 337 CAACTACAAGACCCGCGCCGAG 134. CGCTCCTGGACGTAGCCTTCGGG 135. 24 out 54 G 55 EGFP site 397 GCTGAAGGGCATCGACTTCAAG 136. CCTCGAACTTCACCTCGGCGCGG 137. 25 out 55 G 56 EGFP site 100 CAAGTTCAGCGTGTCCGGCGAG 138. GACCAGGATGGGCACCACCCCGG 139. 26 out 56 G 57 EGFP site 101 AAGTTCAGCGTGTCCGGCGAGG 140. GACCAGGATGGGCACCACCCCGG 141. 27 out 57 G 58 EGFP site 400 GAAGGGCATCGACTTCAAGGAG 142. CCTCGAACTTCACCTCGGCGCGG 143. 28 out 58 G 59 EGFP site 337 CAACTACAAGACCCGCGCCGAG 144. CTGGACGTAGCCTTCGGGCATGG 145. 29 out 59 G 60 EGFP site 307 GGAGCGCACCATCTTCTTCAAG 146. AGAAGTCGTGCTGCTTCATGTGG 147. 31 out 60 G 61 EGFP site 100 CAAGTTCAGCGTGTCCGGCGAG 148. CGTCGCCGTCCAGCTCGACCAGG 149. 10 out 61 G 62 EGFP site 286 CATGCCCGAAGGCTACGTCCAG 150. AGAAGTCGTGCTGCTTCATGTGG 151. 10 out 62 G 63 EGFP site 337 CAACTACAAGACCCGCGCCGAG 152. TGAAGAAGATGGTGCGCTCCTGG 153. 10 out 63 G 64 EGFP site 382 GGTGAACCGCATCGAGCTGAAG 154. CCTCGAACTTCACCTCGGCGCGG 155. 10 out 64 G 65 EGFP site 275 TTCAAGTCCGCCATGCCCGAAG 156. GCTTCATGTGGTCGGGGTAGCGG 157. 11 out 65 G 66 EGFP site 349 CCGCGCCGAGGTGAAGTTCGAG 158. GCCGTCGTCCTTGAAGAAGATGG 159. 11 out 66 G 67 EGFP site 382 GGTGAACCGCATCGAGCTGAAG 160. CTCGAACTTCACCTCGGCGCGGG 161. 11 out 67 G 68 EGFP site 383 GTGAACCGCATCGAGCTGAAGG 162. CCTCGAACTTCACCTCGGCGCGG 163. 11 out 68 G 69 EGFP site 520 CAAGATCCGCCACAACATCGAG 164. GATGCCGTTCTTCTGCTTGTCGG 165. 11 out 69 G 70 EGFP site 383 GTGAACCGCATCGAGCTGAAGG 166. CTCGAACTTCACCTCGGCGCGGG 167. 12 out 70 G 71 EGFP site 415 CAAGGAGGACGGCAACATCCTG 168. TCAGCTCGATGCGGTTCACCAGG 169. 13 out 71 G 72 EGFP site 286 CATGCCCGAAGGCTACGTCCAG 170. GTCGTGCTGCTTCATGTGGTCGG 171. 14 out 72 G 73 EGFP site 415 CAAGGAGGACGGCAACATCCTG 172. CAGCTCGATGCGGTTCACCAGGG 173. 14 out 73 G 74 EGFP site 416 AAGGAGGACGGCAACATCCTGG 174. TCAGCTCGATGCGGTTCACCAGG 175. 14 out 74 G 75 EGFP site 101 AAGTTCAGCGTGTCCGGCGAGG 176. GCCGTCCAGCTCGACCAGGATGG 177. 15 out 75 G 76 EGFP site 286 CATGCCCGAAGGCTACGTCCAG 178. TCGTGCTGCTTCATGTGGTCGGG 179. 15 out 76 G 77 EGFP site 416 AAGGAGGACGGCAACATCCTGG 180. CAGCTCGATGCGGTTCACCAGGG 181. 15 out 77 G 78 EGFP site 417 AGGAGGACGGCAACATCCTGGG 182. TCAGCTCGATGCGGTTCACCAGG 183. 15 out 78 G 79 EGFP site 524 ATCCGCCACAACATCGAGGACG 184. GATGCCGTTCTTCTGCTTGTCGG 185. 15 out 79 G 80 EGFP site 106 CAGCGTGTCCGGCGAGGGCGAG 186. CGTCGCCGTCCAGCTCGACCAGG 187. 16 out 80 G 81 EGFP site 174 CCGGCAAGCTGCCCGTGCCCTG 188. CAGGGTCAGCTTGCCGTAGGTGG 189. 16 out 81 G 82 EGFP site 286 CATGCCCGAAGGCTACGTCCAG 190. CGTGCTGCTTCATGTGGTCGGGG 191. 16 out 82 G 83 EGFP site 417 AGGAGGACGGCAACATCCTGGG 192. CAGCTCGATGCGGTTCACCAGGG 193. 16 out 83 G 84 EGFP site 427 CAACATCCTGGGGCACAAGCTG 194. CGATGCCCTTCAGCTCGATGCGG 195. 13 in 84 G 85 EGFP site 397 GCTGAAGGGCATCGACTTCAAG 196. GTCGCCCTCGAACTTCACCTCGG 197. 20 out 85 G

Surprisingly, the dCas9-FokI protein did not show detectable EGFP disruption activity when co-expressed with any of the 60 gRNA pairs in human U2OS.EGFP cells (FIG. 5E). However, screening of the FokI-dCas9 protein with the same 60 gRNA pairs did reveal EGFP disruption activity on target sites composed of half-sites in the PAM out orientation and with spacer lengths of 13 to 17 bps and of 26 bps (approximately one turn of the DNA helix more than the 13-17 bp spacer lengths) (FIG. 5B). Testing of FokI-dCas9 on an additional 25 target DNA sites with spacer lengths ranging from 10 to 20 bps and with half-sites in the PAM out orientation demonstrated efficient cleavage on targets with spacer lengths of 13 to 18 bps (FIGS. 5C-D). In these experiments, one site each was tested for spacer lengths of 17 or 18 bps and not all sites with a 13 bp spacer length showed activity. Analysis of a subset of successfully targeted sites by T7EI analysis and Sanger sequencing further confirmed the presence of indels at the intended location. Thus FokI-dCas9 can be directed by two appropriately positioned gRNAs to efficiently cleave a full-length target site of interest. For simplicity, the complex of two FokI-dCas9 fusions and two gRNAs are referred to herein as RNA-guided FokI Nucleases (RFNs).

To extend the initial findings with the EGFP reporter gene and to ascertain whether RFNs could be used to perform routine genome editing of endogenous human genes, gRNA pairs were designed for 12 different target sites in nine different human genes (Table 2). Eleven of the 12 RFNs tested introduced indels with high efficiencies (range of 3 to 40%) at their intended target sites in human U2OS.EGFP cells as judged by T7EI assay (Table 2). Similar results were obtained with these same 12 RFN pairs in HEK293 cells (Table 2). Sanger sequencing of successfully targeted alleles from U2OS.EGFP cells revealed the introduction of a range of indels (primarily deletions) at the expected cleavage site (FIG. 5F). The high success rate and high efficiencies of modifications observed in two different human cell lines demonstrate the robustness of RFNs for modifying endogenous human genes.

Example 2d. RFNs Possess Extended Specificities for their Cleavage Sites

To test whether RFNs possess enhanced recognition specificities associated with dimerization, whether these nucleases strictly depend upon the presence of both gRNAs in a pair was examined. In an ideal dimeric system, single gRNAs should not be able to efficiently direct FokI-dCas9-induced indels. To perform an initial test, two pairs of gRNAs directed to two target sites in EGFP were used that had been shown to efficiently direct FokI-dCas9-induced indels to their target sites (EGFP sites 47 and 81) in human U2OS.EGFP cells (FIG. 5C). Replacement of one or the other gRNA in each of these two pairs with a gRNA targeted to an unrelated site in VEGFA resulted in reduction of EGFP disruption activity (FIG. 6A) and reduction of targeted mutations to undetectable levels as judged by T7EI assays (FIG. 6B). Similarly, the effects of using only one of each of the two gRNAs were tested using pairs that efficiently introduce FokI-dCas9-mediated indels in the human APC, MLH1, and VEGFA genes (Table 2) and again observed loss of detectable RFN-induced indels by T7EI assay (FIG. 6C). These results demonstrate that efficient induction of genome editing by an RFN requires two gRNAs with appropriate complementarity to the full-length target site.

Given that the activities of our RFNs depend on the expression of two gRNAs, it was hoped that their mutagenic effects on known off-target sites of one of the single gRNAs in the pair should be negligible. Performing these direct comparisons requires knowing the off-target sites for a monomeric Cas9 nuclease guided by a single gRNA that itself can also serve one of the two gRNAs needed to target a dimeric RFN. Although very few monomeric Cas9 nuclease off-target sites have been defined in the literature, five off-target sites had been previously identified for one of the gRNAs we used to target the dimeric RFN site in the human VEGFA gene (Example 1). Deep sequencing was used to ascertain whether these five off-target sites showed evidence of mutations in cells in which the VEGFA-targeted RFNs had been expressed (these are the same cells we used for the T7EI assay shown in FIG. 6C). The frequency of indel mutations at all five off-target sites was indistinguishable from background (FIG. 6D and Table 3). These results demonstrate that the use of RFNs can essentially eliminate the off-target effects originally induced by Cas9 nuclease and a single gRNA and are consistent with our observation that a single gRNA expressed with FokI-dCas9 does not efficiently induce indels. Although, at present, it is not possible to perform these direct comparisons on additional sites—such experiments will have to await the identification of off-target sites for more single gRNA sites that can also target a half-site for a dimeric RFN, it was concluded that dimeric RFNs have enhanced specificities relative to standard monomeric Cas9 nucleases.

Endogeneous Sequence of RFN SEQ SEQ target sites in U2OS or SEQ Gene left ID right ID 293 cells ID name target sequence NO: target sequence NO: (spacer sequence in lower case) NO: APC1 CCAGAAGTACGAGCGCCGC 198. TGGCAGGTGAGTGAGGCT 199. CCGGGCGGCGCTCGTACTTCTGGccactggg 200. CCGG GCAGG cgagcgtcTGGCAGGTGAGTGAGGCTGCAGG BRCA1 GAATACCCATCTGTCAGCT 201. GGCGGAACCTGAGAGGCG 202. CCGAAGCTGACAGATGGGTATTCtttgacgg 203. TCGG TAAGG ggggtaggGGCGGAACCTGAGAGGCGTAAGG DDB2 AATATTCAAGCAGCAGGCA 204. CTCGCGCAGGAGGCTGCA 205. CCTGTGCCTGCTGCTTGAATATTtccgcctt 206. CAGG GCGGG ttagggtgCTCGCGCAGGAGGCTGCAGCGGG EMX1 CCCAAAGCCTGGCCAGGGA 207. GCCCCACAGGGCTTGAAG 208. CCACTCCCTGGCCAGGCTTTGGGgaggcctg 209. GTGG CCCGG gagtcatgGCCCCACAGGGCTTGAAGCCCGG FANCF- CCCTACTTCCGCTTTCACC 210. GGAATCCCTTCTGCAGCA 211. CCAAGGTGAAAGCGGAAGTAGGGccttcgcg 212. site 1 TTGG CCTGG cacctcatGGAATCCCTTCTGCAGCACCTGG FANCF- CGCTCCAGAGCCGTGCGAA 213. TGGAGGCAAGAGGGCGGC 214. CCCATTCGCACGGCTCTGGAGCGgcggctgc 215. site 2 TGGG TTTGG acaaccagTGGAGGCAAGAGGGCGGCTTTGG FES CGAGGAGACTGGGGACTGT 216. CCAGCTGCTGCCTTGCCT 217. CCCTACAGTCCCCAGCCTCCTCGtcccatgc 218. AGGG CCAGG ctccgtctCCAGCTGCTGCCTTGCCTCCAGG GLI1 CATAGCTACTGATTGGTGG 219. CCGGCCCCTCCCCAGTCA 220. CCCACCACCAATCAGTAGCTATGgcgagccc 221. TGGG GGGGG tgctgtctCCGGCCCCTCCCCAGTCAGGGGG MLH1 GGAAACGTCTAGATGCTCA 222. CAAAATGTCGTTCGTGGC 223. CCGTTGAGCATCTAGACGTTTCCttggctct 224. ACGG AGTGG tctggcgcCAAAATGTCGTTCGTGGCAGGGG RARA1 CTGTTGCTGGCCATGCCAA 225. CCTGGGGGCGGGCACCTC 226. CCGCTTGGCATGGCCAGCAACAGcagctcct 227. GCGG AATGG gcccgacaCCTGGGGGCGGGCACCTCAATGG RUNX TTCGGAGCGAAAACCAAGA 228. GAGTCCCCCGCCTTCAGA 229. CCTGTCTTGGTTTTCGCTCCGAAggtaaaag 230. CAGG AGAGG aaatcattGAGTCCCCCGCCTTCAGAAGAGG SS18 GGCCCGGTCGACTCCGGGC 231. TGCTGGGAATCAGCAGTG 232. CCGGGCCCGGAGTCGACCGGGCCgaggcgga 233. CCGG TTTGG ggcgggccTGCTGGGAATCAGCAGTGTTTGG VEGFA- GGGTGGGGGGAGTTTGCTC 234. TCCCTCTTTAGCCAGAGC 235. CCAGGAGCAAACTCCCCCCACCCcctttcca 236. site 1 CTGG CGGGG aagcccatTCCCTCTTTAGCCAGAGCCGGGG VEGFA- GCCGCCGGCCGGGGAGGAG 237. GGCGAGCCGCGGGCAGGG 238. CCACCTCCTCCCCGGCCGGCGGCggacagtg 239. site 2 GTGG GCCGG gacgcggcGGCGAGCCGCGGGCAGGGGCCGG VEGFA- CCGTCTGCACACCCCGGCT 240. CTCGGCCACCACAGGGAA 241. CCAGAGCCGGGGTGTGCAGACGGcagtcact 242. site 3 CTGG GCTGG agggggcgCTCGGCCACCACAGGGAAGCTGG sizes of cleavage SEQ SEQ DMSO Thermo- amplicon products  Gene primer 1 used ID primer 2 used ID added cycler size in T7E1 name for T7E1 assay NO: for T7E1 assay NO: (yes/no) protocol (bp) (bp) APC1 GGCTGTGGGAAGCCAGCAA 243. AAGCCAGGGGCCA 244. no touchdown 634 447/187 C ACTGGAG BRCA1 GCGCGGGAATTACAGATAA 245. AGGTCCCATCCTC 246. no touchdown 751 454/297 ATTAAAA TCATACATACCA DDB2 ACCGCCCCTTGGCACCAC 247. CGGAGCTCATCTG 248. no touchdown 627 456/171 CTTCCTGT EMX1 GGAGCAGCTGGTCAGAGGG 249. GGGAAGGGGGAC 250. yes two-step 729 480/249 G ACTGGGGA FANCF- GCCCTACATCTGCTCTCCCT 251. GGGCCGGGAAAG 252. no touchdown 634 361/273 site 1 CCA AGTTGCTG FANCF- GCCCTACATCTGCTCTCCCT 253. GGGCCGGGAAAG 254. no touchdown 634 466/168 site 2 CCA AGTTGCTG FES GGGGAGGGAGGCTCCAGGT 255. GGCACAATGGCTC 256. no touchdown 633 395/238 T CCAAGCA GLI1 CCTTACCCCTCCCCTCACTC 257. AGAAGGGCGGGC 258. no touchdown 869 590/279 A CAGACAGT MLH1 ATATCCTTCTAGGTAGCGGG 259. TCTCGGGGGAGAG 260. no touchdown 610 332/278 CAGTAGCC CGGTAAA RARA1 CCCAGGAAAAGTGCCAGCT 261. TGATGGTCACCCC 262. no touchdown 632 335/297 CA AACTGGA RUNX AAGGCGGCGCTGGCTTTTT 263. CCAGCACAACTTA 264. no touchdown 626 389/237 CTCGCACTTGA SS18 GGGATGCAGGGACGGTCAA 265. GCCGCCCCATCCC 266. no touchdown 629 455/174 G TAGAGAAA VEGFA- TCCAGATGGCACATTGTCAG 267. AGGGAGCAGGAA 268. no touchdown 531 338/193 site 1 AGTGAGGT VEGFA- AGAGAAGTCGAGGAAGAGA 269. CAGCAGAAAGTTC 270. yes touchdown 756 482/274 site 2 GAG ATGGTTTCG VEGFA- TCCAGATGGCACATTGTCAG 271. AGGGAGCAGGAA 272. no touchdown 531 288/243 site 3 AGTGAGGT SEQ SEQ Gene primer 1 used ID primer 2 used ID name for deep sequencing NO: for deep sequencing NO: APC1 BRCA1 DDB2 CGATGGCTCCCAAGAAACGC 273. GCAGGTAGAATGCACAGCCG 274. EMX1 FANCF- GCCCAGAGTCAAGGAACACG 275. AGGTAGTGCTTGAGACCGCC 276. site 1 FANCF- CATCCATCGGCGCTTTGGTC 277. CCGGGAAAGAGTTGCTGCAC 278. site 2 FES CTCCCCGTCTGCAGTCCATC 279. CCTGCAGGGACATGTGGTGA 280. GLI1 MLH1 RARA1 RUNX TAGGGCTAGAGGGGTGAGGC 281. CCGAGGTGAAACAAGCTGCC 282. SS18 VEGFA- ATGAGGGCTCCAGATGGCAC 283. TTCACCCAGCTTCCCTGTGG 284. site 1 VEGFA- site 2 VEGFA- site 3

Example 2e. Monomeric Cas9 Nickases Induce Higher Rates of Mutagenesis than Single gRNA/FokI-dCas9 Complexes

As noted above, an important weakness of the paired Cas9 nickase approach is that single monomeric nickases can introduce indel mutations with high frequencies at certain target sites (See Example 1 and Ran et al., Cell 154, 1380-1389 (2013); Mali et al., Nat Biotechnol 31, 833-838 (2013); Cho et al., Genome Res (2013); and Mali et al., Science 339, 823-826 (2013)). This lack of dimerization-dependence in the paired Cas9 nickase system is a potential source of off-target effects because the two monomeric nickases can each create unwanted indel mutations elsewhere in the genome. It was hypothesized that because RFNs introduce alterations using a dimerization-dependent FokI nuclease, these fusions should generally show less undesirable indel activity in the presence of only one gRNA compared to what is observed with monomeric Cas9 nickases.

To test this hypothesis, the activities of FokI-dCas9 and Cas9 nickase were compared in the presence of a single gRNA at six dimeric human gene target sites (a total of 12 half-sites; Table 4). These particular sites were chosen because monomeric Cas9 nickases directed by just one and/or the other gRNA in a pair could induce indel mutations at these targets. Using deep sequencing, the genome editing activities of FokI-dCas9 or Cas9 nickase were assessed in the presence of both or only one or the other gRNAs. Both FokI-dCas9 and Cas9 nickase induced indels at all six target sites with high efficiencies in the presence of two gRNAs (Table 5). As hypothesized, monomeric Cas9 nickases directed by the 12 single gRNAs induced indels with frequencies ranging from 0.0048% to 3.04% (FIG. 7A and Table 5). By contrast, FokI-dCas9 directed by the same 12 single gRNAs induced indels at lower frequencies ranging from 0.0045% to 0.473% (FIG. 7A and Table 5). Comparing these data directly, FokI-dCas9 induced indels with lower frequencies than Cas9 nickase for 10 of the 12 single gRNAs (FIG. 7A and Table 5). In addition, FokI-dCas9 showed greater fold-reductions in indel frequencies than Cas9 nickase at 11 of the 12 half-sites when comparing paired gRNA rates to single gRNA rates (FIG. 7B).

TABLE 4 FokI- dCas9 tdTomoato FokI- FokI- Indel tdTomato Indel Chromo- dCas9 dCas9 Frequency control tdTomato Frequency some Position Site Indels Total (%) indels Total (%) 6 43737290 VEGFA site 1 35000 150158 23.30878 10 258108 0.00387 15 65637531 OT1-3 1 169681 0.00058 1 139847 0.00071 12 131690182 OT1-4 4 190111 0.00210 5 139762 0.00357 12 1988060 OT1-6 3 258976 0.00115 2 178162 0.00112 1 99347645 OT1-11 4 235853 0.00169 4 186287 0.00214 17 39796322 OT1-30 1 261605 0.00038 1 286850 0.00034

TABLE 5 Deep sequencing of FokI-dCas9, Cas9n, and tdTomato controls at 6 sites, with single and pairs of gRNAs (same data as presented in FIG. 7). Nuclease Type Chromo- or Control Site guideRNA some Position Indel Totals Percentages FokI-dCas9 VEGFA site 1 both 6 43737290 35000 150158 23.3088 FokI-dCas9 VEGFA site 1 left 6 43737290 5 95476 0.0052 FokI-dCas9 VEGFA site 1 right 6 43737290 9 91962 0.0098 FokI-dCas9 DDB2 both 11 47236820 11303 50062 22.5780 FokI-dCas9 DDB2 left 11 47236820 311 85726 0.3628 FokI-dCas9 DDB2 right 11 47236820 153 95050 0.1610 FokI-dCas9 FANCF site 1 both 11 22647331 65846 195311 33.7134 FokI-dCas9 FANCF site 1 left 11 22647331 19 27487 0.0691 FokI-dCas9 FANCF site 1 right 11 22647331 845 225154 0.3753 FokI-dCas9 FANCF site 2 both 11 22647138 27743 120314 23.0588 FokI-dCas9 FANCF site 2 left 11 22647138 989 205832 0.4805 FokI-dCas9 FANCF site 2 right 11 22647138 142 165130 0.0860 FokI-dCas9 FES both 15 91428181 14260 125912 11.3254 FokI-dCas9 FES left 15 91428181 4 143877 0.0028 FokI-dCas9 FES right 15 91428181 7 145495 0.0048 FokI-dCas9 RUNX1 both 21 36421217 61057 136164 44.8408 FokI-dCas9 RUNX1 left 21 36421217 222 162636 0.1365 FokI-dCas9 RUNX1 right 21 36421217 109 169122 0.0645 Cas9n VEGFA site 1 both 6 43737290 14294 99036 14.4331 Cas9n VEGFA site 1 left 6 43737290 573 82316 0.6961 Cas9n VEGFA site 1 right 6 43737290 315 101957 0.3090 Cas9n DDB2 both 11 47236820 6673 31168 21.4098 Cas9n DDB2 left 11 47236820 1680 56019 2.9990 Cas9n DDB2 right 11 47236820 172 42424 0.4054 Cas9n FANCF site 1 both 11 22647331 66827 193111 34.6055 Cas9n FANCF site 1 left 11 22647331 1565 109029 1.4354 Cas9n FANCF site 1 right 11 22647331 2457 109289 2.2482 Cas9n FANCF site 2 both 11 22647138 17007 111468 15.2573 Cas9n FANCF site 2 left 11 22647138 120 100591 0.1193 Cas9n FANCF site 2 right 11 22647138 1063 93162 1.1410 Cas9n FES both 15 91428181 16529 126597 13.0564 Cas9n FES left 15 91428181 6 125196 0.0048 Cas9n FES right 15 91428181 23 46102 0.0499 Cas9n RUNX1 both 21 36421217 80029 216800 36.9137 Cas9n RUNX1 left 21 36421217 1106 108670 1.0178 Cas9n RUNX1 right 21 36421217 2169 121413 1.7865 tdTomato VEGF site 1 none 6 43737290 29 313517 0.0092 controls (−) tdTomato FANCF site 1 none 11 22647331 18 578378 0.0031 controls (−) tdTomato FANCF site 2 none 11 22647138 81 393821 0.0206 controls (−) tdTomato FES none 15 91428181 21 410620 0.0051 controls (−) tdTomato DDB2 none 11 47236820 14 165314 0.0085 controls (−) tdTomato RUNX1 none 21 36421217 13 511977 0.0025 controls (−)

The deep sequencing experiments also uncovered a previously undescribed and unexpected side-effect of certain monomeric Cas9 nickases: the introduction of point mutations at particular positions within their target sites. Cas9 nickase co-expressed with a single gRNA for the “right” half-site of the VEGFA target induced base substitutions at position 15 of the recognition site at a frequency of 10.5% (FIG. 8A). Similar results were observed with Cas9 nickase and single gRNAs directed to the “right” half-site of FANCF target site 1 (mutation frequency of 16.3% at position 16) (FIG. 8B) or to the “right” half-site of the RUNX1 target site (mutation frequency of 2% at position 17) (FIG. 8C). Point mutations at these positions were not observed above background levels in control samples in which no Cas9 nickase or gRNA are expressed in the cell (FIGS. 8A-8C). Interestingly, for two of the three sites at which this hypermutation was observed, most of the substitutions observed are C to G transversions on the non-target DNA strand. The positions at which these point mutations were observed fell within a strand-separated region of the target site that has been observed to be susceptible to P1 nuclease in vitro in a dCas9/gRNA/target DNA complex. Importantly, these point mutations occur at much lower frequencies (five to 100-fold lower) in cells that express FokI-dCas9 protein and the same gRNAs (FIG. 8A-C). Overall, it was concluded that FokI-dCas9 nucleases directed by a single gRNA generally induce mutagenic indel and point mutations with lower frequencies than matched single Cas9 nickases.

Example 2f. Dimeric RFNs Possess a High Degree of Specificity

Dimeric RFNs directed by two gRNAs are not expected to induce appreciable off-target mutations in human cells. RFNs, directed by a pair of gRNAs to cleave a full-length sequence composed of two half-sites, would be expected to specify up to 44 bps of DNA in the target site. A sequence of this length will, by chance, almost always be unique (except in certain circumstances where the target might lie in duplicated genome sequence). In addition, the most closely matched sites in the genome to this full-length site should, in most cases, possess a large number of mismatches, which in turn would be expected to minimize or abolish cleavage activity by an RFN dimer. Indeed, all sites in the human genome that bear 0 to 16 mismatches (and that allow for spacers of length 14 to 17 bps) for the 15 full-length sequences successfully targeted with RFNs in this study were identified. This analysis showed that all 15 full-length sequences were unique and that the most closely matched sites in the genome ranged from 7 to 12 mismatches (Table 6). Sites containing this number of mismatches should not be efficiently mutagenized by RFNs and it will be interesting in future studies to confirm this hypothesis. Overall, dimeric RFNs should possess a high degree of specificity in human cells but the ultimate characterization of specificity will await the development of unbiased methods that can comprehensively define RFN specificity across the entire genome.

TABLE 6 Frequencies of candidate FokI-dCas9 off-target sites in the human genome that bear a defined number of mismatches Gene 0 7 8 9 10 11 12 13 14 15 16 APC 1 1 2 16 74 414 2254 BRCA1 1 1 5 20 164 983 DDB2 1 2 7 58 267 1335 EMX1 1 1 2 8 40 175 828 3494 FANCF 1 2 4 44 298 1639 FANCF 1 2 12 79 358 1718 FES 1 3 8 32 191 939 4505 GLI1 1 2 1 7 69 343 1711 MLH1 1 2 5 22 96 643 RARA 1 1 2 8 39 187 698 2849 RUNX1 1 3 25 145 800 SS18 1 1 2 6 39 280 1207 VEGFA-1 1 1 2 3 22 103 543 2676 VEGFA-2 1 4 9 99 447 1675 5608 18599 VEGFA-3 1 3 20 120 623 2783

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Other Embodiments

It is to be understood that while the invention has been described in conjunction with the detailed description thereof, the foregoing description is intended to illustrate and not limit the scope of the invention, which is defined by the scope of the appended claims. Other aspects, advantages, and modifications are within the scope of the following claims. 

What is claimed is:
 1. An RNA-guided FokI Nuclease (RFN) fusion protein comprising SEQ ID NO:26.
 2. A nucleic acid encoding the fusion protein of claim
 1. 3. The nucleic acid of claim 2, comprising SEQ ID NO:27.
 4. A vector comprising the nucleic acid of claim
 2. 5. A host cell expressing the fusion protein of claim
 1. 6. A method of inducing a sequence-specific break in a genomic sequence in a cell, the method comprising expressing in the cell, or contacting the cell with, the RNA-guided FokI Nuclease (RFN) fusion protein of claim 1, and guide RNAs that direct the RFN to two target genomic sequences.
 7. The method of claim 6, wherein the guide RNAs are: (a) two single guide RNAs, wherein one single guide RNA targets a first strand, and the other guide RNA targets the complementary strand, and FokI cuts each strand resulting in a pair of nicks on opposite DNA strands, thereby creating a double-stranded break, or (b) a tracrRNA and two crRNAs wherein one crRNA targets a first strand, and the other crRNA targets the complementary strand, and FokI cuts each strand resulting in a pair of nicks on opposite DNA strands, thereby creating a double-stranded break.
 8. The method of claim 6, wherein each of the two guide RNAs include a complementarity region that is complementary to 17-20 nucleotides of target genomic sequence.
 9. The method of claim 6, wherein an indel mutation is induced between the two target sequences.
 10. The method of claim 6, wherein the two target genomic sequences each have a Protospacer Adjacent Motif (PAM) sequence at the 3′ end.
 11. The method of claim 6, wherein the two target genomic sequences are spaced 0-31 nucleotides apart.
 12. The method of claim 11, wherein the two target genomic sequences are spaced 10-20 base pairs apart.
 13. The method of claim 12, wherein the two target genomic sequences are spaced 13-17 base pairs apart.
 14. A method of increasing specificity of RNA-guided genome editing in a cell, the method comprising contacting the cell with the RNA-guided FokI Nuclease (RFN) fusion protein of claim
 1. 